The Darkhold is a grimoire which consists of the scrolls of Chthon. Hundreds of millions of years ago, when Chthon and the other Elder Gods were corrupted, and began practicing forbidden arts, he recorded his dark knowledge on indestructible parchment. As the coming defeat and annihilation of the Elder Gods at the hands of the Demigorge, a god-eating god created by Gaea to cleanse Earth of their evil Chthon fled into a parallel dimension of his own creation, leaving the Darkhold behind on Earth to provide him a link back to his homeworld.
The chaotic magic of the Darkhold can create disastrous effects across the Earth, and when used by Chthon himself, it can reshape the planet. Its necromantic spells have been used to create vampires, zombies, and werewolves. Various dark cults and sorcerers have used these powers down through the ages, including Atlantean sorcerers who became the world's first vampires.
Certain pages contain spells for summoning demons and monsters or hold imprisoned demons (e.g. Wish-Demon).
The infamous Necronomicon is actually based on the Darkhold, and only contains within it a small fraction of the dark knowledge of the Darkhold.