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What can an Old One do?
Lots and lots. Although the Old Ones don't usually call it "magic", I will call it magic here, since it's easier.
There is a verbal element to a lot of the magic, as well as a physical. Old Ones often raise their hand, pointing with their fingers, as they utter a word or two, usually in the Old Speech. Not all works of magic needs this, though. Often it just needs a thought. Will just needs to think that the fire should go out, and the fire goes out.
Sometimes magic is done with songs. When Will sings an a spell-song in the Old Speech, it doesn't have words like human speech, but is more a matter of nuance and sound.
Will often hears a haunting, magical music when great works of magic happen. This is lilting music, as if played on delicate, unknown instruments, and has a single bell-like phrase through it "in a gold thread of delight." This is particular the case when travelling through Time, but also comes at certain other times of great power.
Sometimes magic happens just by looking hard and something and emptying the mind. When Will is trying to go back to old Caerleon, he gazes at the picture of Caerleon, empties his mind... and gradually finds the reality of the place appearing around him. Later, he is able to jump 1500 years in time just by following an emotion. The Roman soldier's homesickness allows Will to jump to another time, where someone else is feeling homesick.
When an Old One is striking out with full power, he looks like a towering pillar of white light. This is what Simon sees when the Old Ones repel the painter, and Will later looks like this when he is confronting Caradog Pritchard. When he's confronting the Rider in his own home on Christmas day, he feels three times his normal height, towering in rage. Most magic, though, doesn't seem to call for these full measures.
Magic is stronger when on an Old Way. The Old Ways have a magic of their own, that seems to manifest itself in white fire. When Merriman appears on "Tramps' Alley", a circle of white flames burst up around Maggie Barnes, and Merriman says that the power of the Old Ways has awakened.
Knowing someone's name gives you a strong power over them. The birth signs carved for the Stanton children can work in the same way. Merriman drives away Maggie Barnes by knowing her true name, and tells Will that he'll learn the true names of everything soon. Merriman Lyon is not Merriman's true name, by the way - and, presumably, Will also has another, secret, true name.
Totems can be used. The Rider binds Mary by using her birth sign and her hair, and talks about how in the old days people could be bound through some earth where they passed, or even a place their shadow had touched.
Circles are important. When Will breaks the Circle formed by himself, the Lady and Merriman, the Dark can get in, and the Lady expends too much of herself dealing with it. Later, Will exposes the Dark's warestone by drawing a circle with a stick, and speaking in the Old Speech, which causes the circle fo flare up in blue fire.
Some magics can be evaded if someone is invited in. The Rider is able to evade Will's protections on the house because Mr Stanton, the master of the house, invited him in.
Magic - like speaking in the Old Speech - can act as a beacon, and draw the Dark.
No spells last forever, and most only last for "a short breath of time."
Making people forget
This is done by raising a hand, spreading all five fingers, and saying "Forget." It doesn't seem to require any more words than that. The exact amount of forgetting is infinitely controllable. It can mean forgetting one small thing that has just happened, or forgetting a lifetime of experiences. False memories can also be constructed to fill in any gaps. Sometimes this is done verbally, such as when Captain Toms made the children forget that they had seen Will and Merriman leap from the cliff. After he said "forget", they froze for a moment, and he spoke the false memory to them, and told them not to be afraid. When he lowered his hand, they moved again, the new memory implanted.
Forgetfulness is sometimes done in other ways. To make Stephen forget, Will finds (or summons?) some plume moths, which he says have the power to take away memories. As they fly away, Stephen forgets.
Travelling through Time
Merriman explains that an Old One is not rooted in any one period of Time, but can travel backwards and forwards at will. This travelling is often done through the appearance of great wooden doors, and is accompanied with haunting music. These doors are carved with zig-zagging symbols, carved in endless variation on every panel. The wood is "cracked and pitted and yet polished by age," and almost looks like stone, except that there is occasional rounded lumps where a knot hole was. There are no handles.
There are other ways of travelling, too. Will travels to Roman times by sinking into a picture, and Will and Bran travel back in time by turning around while holding Eirias. Other times, the travelling just seems to happen, with Will sinking back into Time without really knowing it.
When Will goes to a party in 1875, Hawkin tells him that if anyone wrote a history of that party, Will would appear in it. He always was there. This is not the sort of time travel that leads to paradoxes in which you kill your father and therefore can't exist.
When the Signs are found in the 1970s, they can then be hidden in the year c. 100, stay there unseen until the present day... and also used by Arthur at Badon, and at the final Rising. Time is very fluid.
The Signs are forged in a bubble of Time, somewhere between "now" and 500 years ago. Bits of sound from both times can be heard.
When Will and the other Old Ones go back in time from the frozen manor, Will sees that the others seem to step out of themselves, leaving one form behind in the "present", while they step into the past.
Seeing into the future?
Most of this is probably caused by the fluid nature of Time to an Old One, by which past, present and future have little real meaning.
There are prophecies in the sequence, some of them uttered by Merriman himself long ago. Are they propechies, or has Merriman just peeked into the future and seen how it will all end? It doesn't seem entirely to be the latter. Merriman is genuinely anxious about how things will turn out, and the result, although prophecied, is not a foregone conclusion. As Will comments, the Dark has their own prophecies.
Merriman can also look into the future and make statements about little things - even things that will happen after he's left Time. For example, he tells James that he will have a very good tenor voice.
Seeing the history of an object
When Will looks into the glass of metheglyn, he gets a glimpse of a group of monks making the liquid he has just drunk.
Slightly similar, after Bran has broken the power of the warestone, Will is able to find out what happened. He holds the warestone, says words in the Old Speech, and empties his mind to receive whatever awareness the stone will put into it. "The knowledge would not be simple and open, he knew. It never was." The stone shows him images from the whole story of Bran coming to the modern world, and not just the moment that Bran broke the control of the stone.
Listening
When Will wants to be able to hear what Hawkin and Maggie Barnes are saying, Merriman reminds him that he only needs to wish to hear them, and he can.
Freezing people in Time
An Old One can do this merely by pointing at someone with the fingers spread. No words are used. Someone is frozen exactly where they stood, and remain unaware of everything, until the spell is broken.
Disappearing
Merriman causes the Walker to disappear, becoming first transparent, then vanishing "like a star blotted out by a cloud." Merriman says he's sent the Walker to another place to have peace. He says that this is the power of the Old Ways, and that Will could have used the trick himself to escape, had he known how. It's not clear how much this can be done when not on an Old Way, though.
Speaking into someone else's mind
Old Ones can speak into each other's mind, silently, so no-one else can hear. It's not clear how far away this works. Do they have to be close? Do they have to see each other? Can they do this to someone who isn't an Old One? That seems unlikely. Merriman tells Will that the Dark has ways of hearing speech done this way, though, and they need to be careful.
Putting images into someone else's mind
During their first meeting, Merriman puts an image into Will's mind, and Will does the same to Merriman.
In Wales, Will gets a quick glimpse of an image that he knows was sent to him from his masters far away. He calls this a "very rare" occurrence - rare, presumably, in that it happened despite his masters being far away. Right at the start, we learn that Old Ones can put images into each other's head effortlessly, when near by.
Under Bird Rock, Will thinks that he can't help Bran with the riddle, because in this place the law of High Magic means that he can't put an image or thought into another's mind. This does imply, though, that normally he would be able to do this.
Later, Will puts into Caradog Pritchard's mind an illusion of the lake flooding.
Seeing into someone else's mind
Although Old Ones can't see into the minds of mortals, because that would be bad, it seems as if they can see into the minds of their enemies, at least sometimes. When the Rider comes to the Stanton family home at Christmas, Will tries to see into his mind, to guess his intentions, but comes against a black wall. Still, the fact that he tries suggests that this ought to have worked, perhaps with a lesser enemy. He also suspects that the Rider can see into his own mind, but thinks he can only read the surface thoughts.
Control the elements
When Will is reading the book of Gramarye, he learns the words that will control "wind and storm, sky and air, cloud and rain, and snow and hail - and everything in the sky save the sun and the moon, the planets and the stars." (Later, though, he flies through the stars and planets, and knows "every spell of the sun and the moon.") He then learns the spells of sea, river, stream, lake, beck and fjord - but also learns that water is the one element that can resist magic, for no magic works in running water.
Speaking all languages
An Old One can speak all the languages of men. See Why doesn't Will know Welsh? They also speak the Old Speech. Will knows this instantly as soon as he awakens to his power, and speaks it without knowing that he is speaking it.
Transporting objects
When Will rushes back to get the harp, he thinks how much easier it would be if he was Merriman. Merriman can effortlessly transport an object through space and even Time - i.e. Merriman could presumably just think, "I wish the harp was here", and it would be here. Will can't do this..."yet." In fanfic set in the future, though...?
Affecting objects
Will's first act of magic is to put out a fire just by thinking about it going out. He then learns how to cause a fire to burn, using the same method. Merriman tells him to envisage the result he wants, and it will happen.
Sensing the Dark
Will says that all the Old Ones can sense the Dark whenever they are near. (Owain Glyndwr appears not to be able to do this, though.) He senses the afanc just before it appears.
Sensing the Old Ways
John Rowlands says that Will could probably walk blindfold along the entire Old Way, now that he's found it. Will agrees.
Riding the wind
When John Rowlands first sees Merriman, he is riding the wind. This is mentioned as a skill Will learns in the The Book of Gramarye, though we don't see him using it - unless we count diving off the cliff in Greenwitch, and seeming to hang in the air for a while before falling.
Running very fast
When Will and Merriman run towards the cliff in Trewissick, the children see them running like swift animals, "an urgent loping running that took away their age, and all sense of familiarity in their appearance; faster, faster, faster."
Swimming like a fish
Will and Merriman dive under water, breathing as fishes do. They go incredibly deep, far deeper than any man could survive, and talk to Tethys.
Flying amongst the stars
When reading the Book of Gramarye, Will flies among the stars. Later he thinks of each individual star and constellation as "his friends", and greets them mentally, remembering how he had flown amongst them.
Protections
Will puts a protection around Paul and the vicar, so that no living thing can affect them, and they cannot see anything outside the barrier. It is dangerous, because they will be trapped forever behind it if Will is prevented from lifting it. Earlier, he throws up a barrier around Paul, himself and Merriman, when the Rider is approaching them in the Manor, while Paul is playing the flute.
In Wales, Will uses a spell to protect himself and Bran as they go through the Grey King's domain. This spell protects a wandered from harm as he walks through someone else's land.
Will puts a spell on the harp, hiding it so that no-one else can see it, or remove it. Only the song of an Old One can break the spell.
Break the power of the Dark
When "Mr Mitothin" comes to visit on Christmas Day, Will gropes in his memory for the words of destruction that "in the last resort" can break the power of the Dark. However, the Rider tells him that he can't use such weapons here, or his whole family will be blasted out of Time.
Play music
Will says he could play the golden harp "by enchantment."
What can't an Old One do?
Some of these things are not possible with magic in this world. Others are possible, but only for the Dark, since the Light cannot interfere with human free choice.
Mind reading
An Old One cannot read a mortal's mind. Will tells John Rowlands that an Old One - a normal one, anyway - can't know things about men that they haven't been told or see with their own eyes. He knows things about the universe and about magic, but not about the secrets of mortal men.
Changing things
We never see an Old One turning a person into a newt, or a pen into a penguin. Magic does not change one thing to another thing.
Creating things
Magic can cause something to burst into flame, but there no cases of an Old One being cold, and magicking up a blanket, or whatever. All the Things of Power were made by craftsmen.
Teleport things
Will, on his first quest alone, has to travel the human way to get the harp. Merriman, he thinks, could have just brought the harp to his side using magic. Will can't do this "yet."
Interfering with the Wild Magic
The Wild Magic cannot take sides, but neither can Light nor Dark interfere with the Wild Magic. Merriman must bargain with Tethys, and the Old Ones cannot compel the Greenwitch to hand over her secret. Will and the Dark are both fairly powerless in the Lost Land.
Interfere with a warestone that belongs to someone else
If the Dark has used a warestone (see Warestones), no Old One can break its spell, and vice versa.
Possess someone
The Dark can do this. The Grey King channels his powers through Caradog Pritchard, though this is described as very dangerous, since there is the risk that the mortal mind will break. The Light cannot do this. Or, rather, technically they can, but they cannot do this and remain of the Light. Since they unequivocably are of the Light - they cannot ever change this because it is their nature - this does effectively mean that they cannot ever possess someone. The Light is committed to preserving human free will.
Control someone's mind
The Dark does this, but the Light does not. The Rider taunts Will, saying that the Old Ones have neglected parts of the craft. Although they can't outright kill someone, the Dark can influence someone so they go and throw themselves in a river. The Light can't.
Use strong magic when mortals are present
When Will tries to use strong magic against the Rider on Christmas Day, the Rider says that he can't do this, because his family are present. Such strong magic will blast his family out of Time forever.
Kill people, or directly harm them in any way
Despite what John Rowlands says, the Light cannot do anything that directly harms a human. They prize human choice. Will does use fairly strong magic on Caradog Pritchard, but only after he has refused many pleas to turn aside, and only because Pritchard is hardly innocent now, but serving the Dark.
Kill the Dark
The lords of the Dark are immortal, just as the Old Ones are. They cannot kill each other, though they can blast each other out of Time far enough that they won't be able to return until it's too late. "None but the Dark can destroy the Dark," Will tells the White Rider.
Do magic with water
Water is the one element that can defy all magic. Moving water doesn't tolerate any magic, whether good or evil, and will wash it away as if it has never been made.
Rely on mortals
Well, that can, but they shouldn't. When Hawkin listens to Maggie Barnes, Merriman says that he made the worst mistake than an Old One could make - to put more trust in a mortal man than he has the strength to take. "It is something that all of us learned never to do, centuries ago."
Resist the enemy when it has been invited in
Will is limited in what he can do against the Rider because his father has invited "Mr Mitothin" into the house. When the Walker calls the Dark into the Manor, there is little that the Light can do to stop it.
Touch someone sworn to the other side
When Hawkin is liege man to Merriman, the Dark cannot touch him, but when he swears to the Dark, the Light cannot interfere with him, either.
Make a man more than a man
Merriman says to Hawkin that no power of Dark or Light can make a man more than a man, once any supernatural role he may have had to play comes to and end. And no power of Light or Dark can take away his rights as a man, either. Man is always free to choose his fate.
Old Ones can also be put out of Time, in a way, by the Dark. Near the end of The Dark is Rising, Will is mesmerised by a tiny twig, so he's unaware of the fact that the Dark is going magic next to him. He thinks of it as the Dark putting an Old One out of Time for a psace, "if they needed a space for their own magic." This is done with high wordless singing
What can an Old One do?
Lots and lots. Although the Old Ones don't usually call it "magic", I will call it magic here, since it's easier.
There is a verbal element to a lot of the magic, as well as a physical. Old Ones often raise their hand, pointing with their fingers, as they utter a word or two, usually in the Old Speech. Not all works of magic needs this, though. Often it just needs a thought. Will just needs to think that the fire should go out, and the fire goes out.
Sometimes magic is done with songs. When Will sings an a spell-song in the Old Speech, it doesn't have words like human speech, but is more a matter of nuance and sound.
Will often hears a haunting, magical music when great works of magic happen. This is lilting music, as if played on delicate, unknown instruments, and has a single bell-like phrase through it "in a gold thread of delight." This is particular the case when travelling through Time, but also comes at certain other times of great power.
Sometimes magic happens just by looking hard and something and emptying the mind. When Will is trying to go back to old Caerleon, he gazes at the picture of Caerleon, empties his mind... and gradually finds the reality of the place appearing around him. Later, he is able to jump 1500 years in time just by following an emotion. The Roman soldier's homesickness allows Will to jump to another time, where someone else is feeling homesick.
When an Old One is striking out with full power, he looks like a towering pillar of white light. This is what Simon sees when the Old Ones repel the painter, and Will later looks like this when he is confronting Caradog Pritchard. When he's confronting the Rider in his own home on Christmas day, he feels three times his normal height, towering in rage. Most magic, though, doesn't seem to call for these full measures.
Magic is stronger when on an Old Way. The Old Ways have a magic of their own, that seems to manifest itself in white fire. When Merriman appears on "Tramps' Alley", a circle of white flames burst up around Maggie Barnes, and Merriman says that the power of the Old Ways has awakened.
Knowing someone's name gives you a strong power over them. The birth signs carved for the Stanton children can work in the same way. Merriman drives away Maggie Barnes by knowing her true name, and tells Will that he'll learn the true names of everything soon. Merriman Lyon is not Merriman's true name, by the way - and, presumably, Will also has another, secret, true name.
Totems can be used. The Rider binds Mary by using her birth sign and her hair, and talks about how in the old days people could be bound through some earth where they passed, or even a place their shadow had touched.
Circles are important. When Will breaks the Circle formed by himself, the Lady and Merriman, the Dark can get in, and the Lady expends too much of herself dealing with it. Later, Will exposes the Dark's warestone by drawing a circle with a stick, and speaking in the Old Speech, which causes the circle fo flare up in blue fire.
Some magics can be evaded if someone is invited in. The Rider is able to evade Will's protections on the house because Mr Stanton, the master of the house, invited him in.
Magic - like speaking in the Old Speech - can act as a beacon, and draw the Dark.
No spells last forever, and most only last for "a short breath of time."
Making people forget
This is done by raising a hand, spreading all five fingers, and saying "Forget." It doesn't seem to require any more words than that. The exact amount of forgetting is infinitely controllable. It can mean forgetting one small thing that has just happened, or forgetting a lifetime of experiences. False memories can also be constructed to fill in any gaps. Sometimes this is done verbally, such as when Captain Toms made the children forget that they had seen Will and Merriman leap from the cliff. After he said "forget", they froze for a moment, and he spoke the false memory to them, and told them not to be afraid. When he lowered his hand, they moved again, the new memory implanted.
Forgetfulness is sometimes done in other ways. To make Stephen forget, Will finds (or summons?) some plume moths, which he says have the power to take away memories. As they fly away, Stephen forgets.
Travelling through Time
Merriman explains that an Old One is not rooted in any one period of Time, but can travel backwards and forwards at will. This travelling is often done through the appearance of great wooden doors, and is accompanied with haunting music. These doors are carved with zig-zagging symbols, carved in endless variation on every panel. The wood is "cracked and pitted and yet polished by age," and almost looks like stone, except that there is occasional rounded lumps where a knot hole was. There are no handles.
There are other ways of travelling, too. Will travels to Roman times by sinking into a picture, and Will and Bran travel back in time by turning around while holding Eirias. Other times, the travelling just seems to happen, with Will sinking back into Time without really knowing it.
When Will goes to a party in 1875, Hawkin tells him that if anyone wrote a history of that party, Will would appear in it. He always was there. This is not the sort of time travel that leads to paradoxes in which you kill your father and therefore can't exist.
When the Signs are found in the 1970s, they can then be hidden in the year c. 100, stay there unseen until the present day... and also used by Arthur at Badon, and at the final Rising. Time is very fluid.
The Signs are forged in a bubble of Time, somewhere between "now" and 500 years ago. Bits of sound from both times can be heard.
When Will and the other Old Ones go back in time from the frozen manor, Will sees that the others seem to step out of themselves, leaving one form behind in the "present", while they step into the past.
Seeing into the future?
Most of this is probably caused by the fluid nature of Time to an Old One, by which past, present and future have little real meaning.
There are prophecies in the sequence, some of them uttered by Merriman himself long ago. Are they propechies, or has Merriman just peeked into the future and seen how it will all end? It doesn't seem entirely to be the latter. Merriman is genuinely anxious about how things will turn out, and the result, although prophecied, is not a foregone conclusion. As Will comments, the Dark has their own prophecies.
Merriman can also look into the future and make statements about little things - even things that will happen after he's left Time. For example, he tells James that he will have a very good tenor voice.
Seeing the history of an object
When Will looks into the glass of metheglyn, he gets a glimpse of a group of monks making the liquid he has just drunk.
Slightly similar, after Bran has broken the power of the warestone, Will is able to find out what happened. He holds the warestone, says words in the Old Speech, and empties his mind to receive whatever awareness the stone will put into it. "The knowledge would not be simple and open, he knew. It never was." The stone shows him images from the whole story of Bran coming to the modern world, and not just the moment that Bran broke the control of the stone.
Listening
When Will wants to be able to hear what Hawkin and Maggie Barnes are saying, Merriman reminds him that he only needs to wish to hear them, and he can.
Freezing people in Time
An Old One can do this merely by pointing at someone with the fingers spread. No words are used. Someone is frozen exactly where they stood, and remain unaware of everything, until the spell is broken.
Disappearing
Merriman causes the Walker to disappear, becoming first transparent, then vanishing "like a star blotted out by a cloud." Merriman says he's sent the Walker to another place to have peace. He says that this is the power of the Old Ways, and that Will could have used the trick himself to escape, had he known how. It's not clear how much this can be done when not on an Old Way, though.
Speaking into someone else's mind
Old Ones can speak into each other's mind, silently, so no-one else can hear. It's not clear how far away this works. Do they have to be close? Do they have to see each other? Can they do this to someone who isn't an Old One? That seems unlikely. Merriman tells Will that the Dark has ways of hearing speech done this way, though, and they need to be careful.
Putting images into someone else's mind
During their first meeting, Merriman puts an image into Will's mind, and Will does the same to Merriman.
In Wales, Will gets a quick glimpse of an image that he knows was sent to him from his masters far away. He calls this a "very rare" occurrence - rare, presumably, in that it happened despite his masters being far away. Right at the start, we learn that Old Ones can put images into each other's head effortlessly, when near by.
Under Bird Rock, Will thinks that he can't help Bran with the riddle, because in this place the law of High Magic means that he can't put an image or thought into another's mind. This does imply, though, that normally he would be able to do this.
Later, Will puts into Caradog Pritchard's mind an illusion of the lake flooding.
Seeing into someone else's mind
Although Old Ones can't see into the minds of mortals, because that would be bad, it seems as if they can see into the minds of their enemies, at least sometimes. When the Rider comes to the Stanton family home at Christmas, Will tries to see into his mind, to guess his intentions, but comes against a black wall. Still, the fact that he tries suggests that this ought to have worked, perhaps with a lesser enemy. He also suspects that the Rider can see into his own mind, but thinks he can only read the surface thoughts.
Control the elements
When Will is reading the book of Gramarye, he learns the words that will control "wind and storm, sky and air, cloud and rain, and snow and hail - and everything in the sky save the sun and the moon, the planets and the stars." (Later, though, he flies through the stars and planets, and knows "every spell of the sun and the moon.") He then learns the spells of sea, river, stream, lake, beck and fjord - but also learns that water is the one element that can resist magic, for no magic works in running water.
Speaking all languages
An Old One can speak all the languages of men. See Why doesn't Will know Welsh? They also speak the Old Speech. Will knows this instantly as soon as he awakens to his power, and speaks it without knowing that he is speaking it.
Transporting objects
When Will rushes back to get the harp, he thinks how much easier it would be if he was Merriman. Merriman can effortlessly transport an object through space and even Time - i.e. Merriman could presumably just think, "I wish the harp was here", and it would be here. Will can't do this..."yet." In fanfic set in the future, though...?
Affecting objects
Will's first act of magic is to put out a fire just by thinking about it going out. He then learns how to cause a fire to burn, using the same method. Merriman tells him to envisage the result he wants, and it will happen.
Sensing the Dark
Will says that all the Old Ones can sense the Dark whenever they are near. (Owain Glyndwr appears not to be able to do this, though.) He senses the afanc just before it appears.
Sensing the Old Ways
John Rowlands says that Will could probably walk blindfold along the entire Old Way, now that he's found it. Will agrees.
Riding the wind
When John Rowlands first sees Merriman, he is riding the wind. This is mentioned as a skill Will learns in the The Book of Gramarye, though we don't see him using it - unless we count diving off the cliff in Greenwitch, and seeming to hang in the air for a while before falling.
Running very fast
When Will and Merriman run towards the cliff in Trewissick, the children see them running like swift animals, "an urgent loping running that took away their age, and all sense of familiarity in their appearance; faster, faster, faster."
Swimming like a fish
Will and Merriman dive under water, breathing as fishes do. They go incredibly deep, far deeper than any man could survive, and talk to Tethys.
Flying amongst the stars
When reading the Book of Gramarye, Will flies among the stars. Later he thinks of each individual star and constellation as "his friends", and greets them mentally, remembering how he had flown amongst them.
Protections
Will puts a protection around Paul and the vicar, so that no living thing can affect them, and they cannot see anything outside the barrier. It is dangerous, because they will be trapped forever behind it if Will is prevented from lifting it. Earlier, he throws up a barrier around Paul, himself and Merriman, when the Rider is approaching them in the Manor, while Paul is playing the flute.
In Wales, Will uses a spell to protect himself and Bran as they go through the Grey King's domain. This spell protects a wandered from harm as he walks through someone else's land.
Will puts a spell on the harp, hiding it so that no-one else can see it, or remove it. Only the song of an Old One can break the spell.
Break the power of the Dark
When "Mr Mitothin" comes to visit on Christmas Day, Will gropes in his memory for the words of destruction that "in the last resort" can break the power of the Dark. However, the Rider tells him that he can't use such weapons here, or his whole family will be blasted out of Time.
Play music
Will says he could play the golden harp "by enchantment."
What can't an Old One do?
Some of these things are not possible with magic in this world. Others are possible, but only for the Dark, since the Light cannot interfere with human free choice.
Mind reading
An Old One cannot read a mortal's mind. Will tells John Rowlands that an Old One - a normal one, anyway - can't know things about men that they haven't been told or see with their own eyes. He knows things about the universe and about magic, but not about the secrets of mortal men.
Changing things
We never see an Old One turning a person into a newt, or a pen into a penguin. Magic does not change one thing to another thing.
Creating things
Magic can cause something to burst into flame, but there no cases of an Old One being cold, and magicking up a blanket, or whatever. All the Things of Power were made by craftsmen.
Teleport things
Will, on his first quest alone, has to travel the human way to get the harp. Merriman, he thinks, could have just brought the harp to his side using magic. Will can't do this "yet."
Interfering with the Wild Magic
The Wild Magic cannot take sides, but neither can Light nor Dark interfere with the Wild Magic. Merriman must bargain with Tethys, and the Old Ones cannot compel the Greenwitch to hand over her secret. Will and the Dark are both fairly powerless in the Lost Land.
Interfere with a warestone that belongs to someone else
If the Dark has used a warestone (see Warestones), no Old One can break its spell, and vice versa.
Possess someone
The Dark can do this. The Grey King channels his powers through Caradog Pritchard, though this is described as very dangerous, since there is the risk that the mortal mind will break. The Light cannot do this. Or, rather, technically they can, but they cannot do this and remain of the Light. Since they unequivocably are of the Light - they cannot ever change this because it is their nature - this does effectively mean that they cannot ever possess someone. The Light is committed to preserving human free will.
Control someone's mind
The Dark does this, but the Light does not. The Rider taunts Will, saying that the Old Ones have neglected parts of the craft. Although they can't outright kill someone, the Dark can influence someone so they go and throw themselves in a river. The Light can't.
Use strong magic when mortals are present
When Will tries to use strong magic against the Rider on Christmas Day, the Rider says that he can't do this, because his family are present. Such strong magic will blast his family out of Time forever.
Kill people, or directly harm them in any way
Despite what John Rowlands says, the Light cannot do anything that directly harms a human. They prize human choice. Will does use fairly strong magic on Caradog Pritchard, but only after he has refused many pleas to turn aside, and only because Pritchard is hardly innocent now, but serving the Dark.
Kill the Dark
The lords of the Dark are immortal, just as the Old Ones are. They cannot kill each other, though they can blast each other out of Time far enough that they won't be able to return until it's too late. "None but the Dark can destroy the Dark," Will tells the White Rider.
Do magic with water
Water is the one element that can defy all magic. Moving water doesn't tolerate any magic, whether good or evil, and will wash it away as if it has never been made.
Rely on mortals
Well, that can, but they shouldn't. When Hawkin listens to Maggie Barnes, Merriman says that he made the worst mistake than an Old One could make - to put more trust in a mortal man than he has the strength to take. "It is something that all of us learned never to do, centuries ago."
Resist the enemy when it has been invited in
Will is limited in what he can do against the Rider because his father has invited "Mr Mitothin" into the house. When the Walker calls the Dark into the Manor, there is little that the Light can do to stop it.
Touch someone sworn to the other side
When Hawkin is liege man to Merriman, the Dark cannot touch him, but when he swears to the Dark, the Light cannot interfere with him, either.
Make a man more than a man
Merriman says to Hawkin that no power of Dark or Light can make a man more than a man, once any supernatural role he may have had to play comes to and end. And no power of Light or Dark can take away his rights as a man, either. Man is always free to choose his fate.
Old Ones can also be put out of Time, in a way, by the Dark. Near the end of The Dark is Rising, Will is mesmerised by a tiny twig, so he's unaware of the fact that the Dark is going magic next to him. He thinks of it as the Dark putting an Old One out of Time for a psace, "if they needed a space for their own magic." This is done with high wordless singing
