Native American myth
The Native Americans living near what would become the town of Bright Falls believed that the lake was a way to the realm of the dead.
The Dark Presence normally "sleeps" somewhere at the bottom of Cauldron Lake, in a mysterious location called the "Dark Place". Originally its power was limited: it was trapped and could only slightly alter reality and take over the bodies of some people. The lake itself has the ability to turn works of art created in its proximity to reality, and the Presence, whatever it is, seeks to escape its imprisonment there by using the lake's power. It will awaken when it senses someone with the right amount of creative potential, and attempt to get them to create something that will release it. It also attempts to manipulate the story itself to get itself free from the fetters of the lake. Since a story must stay true to it's internal logic, it can't just be as simple as "I get free. The end.", since that would not be art. This sort of corner-cutting also doesn't work if the artist tries to just have an angel show up and stab it with a flaming sword, for example. That would ruin the suspension of disbelief.
The first known occurrence of this sort was when the poet Thomas Zane came to the island in the middle of Cauldron Lake with his love, Barbara Jagger. As he wrote his poems on the island, he came to realize the lake's amazing abilities, and experimented with them with the help of his friend, Emil Hartman. One day, when his beloved Barbara dove to the bottom of the lake and accidentally drowned, the devastated poet, at Hartman's advice, vowed to write her back to life. However, since he did not specify the manner of her return, she was taken over by the Darkness and came to be used as its avatar from that day forward. Realizing his mistake, Zane wrote himself, his poems, and the entire island out of existence, thus sealing the Presence back.
30 years later, the writer Alan Wake came to the nearby town of Bright Falls with his wife Alice on vacation. The Presence, in the guise of the old widow Barbara Jagger, tricked him into staying on the island (which had mysteriously re-appeared). Once there, it abducted Alice and, as Jagger, convinced Alan that she had died, recommending that he write a novel to bring her back to life. Jagger acted as his editor, altering the novel and turning it from a thriller into a horror story, giving itself ever more power, with the final purpose of having it end with it escaping and consuming the whole world.
The minions of the Dark Presence are known as the Taken. The Dark Presence engulfs a human with a tendril of itself and erases it's personality and free will, and manipulates them to attack the Protagonist of the now-real story. Normal taken are armed with a variety of implements, such as sickles and meathooks. Super Taken are unarmed, but they can move very fast. Chainsaw taken are, of course, armed with chainsaws and are very bulky with a large amount of upper body strength.
In the later stages of the story, as it grows ever more powerful, it begins to take over inanimate objects as well, such as pipes, barrels, carts, and even large vehicles like bulldozers, woodcutters and combine harvesters.
It also incorporates avatars, such as the aforementioned Barbara Jagger and, if the ending is to be believed, Agent Nightingale.