The Dark is a force of the universe, operating in direct opposition to the Light. The task of the Dark is to rise and create a world in which joy, love and laughter can not exist. It is difficult to say, from the book, how much the Dark is sentient, able to think and act and desire for itself, and how much it is simply an idea or an influence that causes its allies to do what they think they should do. Through the eons-long struggle, both sides gain and lose ground at various times, but neither can be completely defeated until the last great conflict, which occurs at the end of the fifth book, Silver on the Tree.
The Dark is hard and ruthless, showing no mercy even to its own, and often after defeating an ally of the Dark, the Light simply lets them go, knowing the Dark will punish them for their failure.
It is important to note that the Light and the Dark are not the only forces at work in the universe; there is also Wild Magic, the magic of the natural world, and High Magic, which governs the universe. All four forces are governed by rigid laws and exist separately from each other, although certain of their laws and customs involve more than one kind (for instance, the challenge in the fifth book, in which the Dark and the Light both argue sides of a case; once a decision is made, it is ratified by the High Magic). There are also places, such as the Lost Land, in which neither Dark nor Light has power. There is a final element at work on Earth, and that is men. Men are inherently neither of the Light nor the Dark, although they can have good or evil in their hearts; the Dark and Light can use these inclinations to their advantage.
The Dark is described metaphorically as a great bottomless pit. It manifests itself in fear, doubt, and anger.
Methods and Nature
The Dark cannot directly physically harm anyone but its own, so it must try to influence someone else to do the harm. Similarly, the Dark cannot force anyone to join it or do its will; it can simply tempt, influence, or frighten. It can do this strongly, as in when it sends a lake monster to frighten Jane Drew, or subtly, as when it sends doubts and fears so carefully that most who feel it assume it is an idea coming from their own mind. To one who is sensitive or who knows what he or she is looking for, this Dark influence is easily discerned as unnaturally strong feelings of fear or anger. For an Old One or another who is similarly strong and well-fortified, these attacks can come in the form of incredibly oppressive fear and darkness, or in a terrifying cacophony of unidentified but terrible sounds and creatures.
Another of its favored methods is to have its agents attempt to convince those who know about it that it doesn't exist, as Mr. Hastings and Polly Withers frequently do to the Drew children in the first book.
To those who are receptive, though, the Dark often tempts instead of terrifies. Its agents can take kind and pleasing human forms, who paint a picture of the Dark as something that can give you what you want. In this method it lures Hawkin, liege-man and adopted son of Merriman Lyon, into betraying the Light. After being deeply shaken by the fact that his life was endangered by Merriman, Hawkin is led to believe by allies of the Dark that if he helps the Dark, they will help him and care for him and never treat him as the Light did. This, however, is a lie, as the Dark has no love for its allies, and Hawkins' betrayal leads him to even more anguish.
When the Dark is not trying to influence a person, though, it can be very inert and undetectable. An example of this is the valley where the fourth book occurs, where Bran Davies and Will Stanton's aunt and uncle live. The powerful Dark Grey King dwells near and watches over the valley, jealously guarding several of its important secrets. However, despite having several characters in the valley of unusual power or perception, no one ever notices the Dark power in the area until Will's arrival sets off a chain of events that angers the Grey King.
Warning: this section contains spoilers.
The Dark has been behind every invading force in Britain; Owain Glyndŵr states in the fifth book that it is been behind the Norman and the Saxon and the Dane. With every invasion, the people who still hold to the old ways of the Light have been pushed farther west, into Wales and Cornwall. Each invading force, however, comes in time to be tamed by the land of Britain, and many descendants of the invaders live in peace and the ways of the Light until another invading force arrives. Though it is never explicitly stated, it seems likely that the the Dark has been behind invasions and wars in other parts of the world as well.
The Dark is destined to have two great risings: the first is in the time of Arthur, which can be assumed to be around the time of the Germanic invasions of Britain a few centuries after the fall of Rome. Arthur, a good and beloved king, is able to defeat the invading forces at the Battle of Mons Badonicus with the help of Merlin (Merriman Lyon) and the Six Signs of the Light gathered by Will Stanton. The Dark is not completely defeated, though, and the peace that Arthur brings lasts only a few centuries.
The second great rising is built up to all throughout the series and finally climaxes at the end of the fifth book. The Dark is defeated, and both Light and Dark, and all the immortal creatures who have aided in the fight, leave the earth, leaving the fate of the world in the hands of men.
The Dark has many allies and agents who do its bidding and seek to bring about it final end: a world enslaved by darkness.
The Dark's most powerful agents are the supernatural beings who aid it. One of the most powerful of these is the Grey King, who dwells in Wales, watching over the resting place of the Sleepers. Still others include supernatural beasts such as the afanc (lake monster) who confronts Jane in the fifth book.
Many other allies of the Dark display supernatural abilities, but it is difficult to say whether they are actually supernatural beings. An example of this is the Black and White Riders, two of the great lords of the Dark. This will be addressed in the next section.
The Dark frequently makes allies out of men through temptation and preying on their darker feelings and tendencies. These allies fall into two kinds: those who realize what they're doing and those who don't. Many, such as Norman Withers and Polly Withers, are average humans who have willingly and knowingly chosen to follow the Dark, likely with the promise of rewards to come. Still others, such as Caradog Prichard, unwittingly become allies of the Dark when their own negative emotions and dark urges make them susceptible; these people become channels for the Dark, believing they are acting of their own volition when really they are responding to the Dark.
As mentioned above, it is often hard to tell whether an agent of the Dark is human or simply taking human form. The Light, similarly, has great allies called Old Ones; these beings are like human in many ways, being born as humans and having human appearances, but they are immortal and have great power. It is never explicitly stated whether the Dark has similar beings. Many of its allies are human in appearance but supernatural in powers; these include the painter in the third book who looks human but is able to work spells and, more importantly, the Black and White Riders. It is unknown whether these two Riders are human or not. The White Rider, at least, lived many years on Earth as a human, but when it is asked whether she was a human who chose the Dark or she was a Dark creature all along, no definitive answer is given. It is also clearly shown that the Dark often gives its mortal allies some of its power, such as when the Grey King uses Caradog Pritchard to try to stop Will from waking the Sleepers. It is therefore uncertain whether beings such as the very powerful Black Rider are supernatural in origin or simply men who have turned so completely, and the Dark has given them so much power, that they now appear to be supernatural.
Some of the allies of the Dark are animals. The most frequently mentioned are rooks. These birds frequently act as spies for the Dark and occasionally even attack allies of the Light, especially in the second book. However, these animals seem to have some ability to choose whether to follow the Dark; Merriman comments that wiser birds do not join the Dark.
There are also polecats and minks, animals that kill for pleasure and are often used by the Dark to intimidate or attack allies of the Light, especially in the last book. It is unknown whether the author is implying that these animals' natural inclination to kill for sport makes them natural allies for the Dark, or they have allied themselves with the Dark and are therefore more inclined to kill for sport.