An ancient set of tomes created by dark wizards of Atlantis in conjunction with the malevolent entity known as Chthon. The spells in the Darkhold are universally wicked-- rituals designed to control, to kill, to manipulate. They are the blackest of magics and the book is dangerously easy for even unpracticed wizards to read and study. Single pages from the tome have been known to alter the course of history.
The Darkholde is sufficiently potent that it is nearly impossible to destroy. It's an ideologue in its own right and will reassemble itself over time. Merlyn used the Tesseract to scatter the pages of the Darkhold through the realms of the Demiurge, and entrusted the energy matrix to an incarnation of Odin early in Asgard's cycle of the Ragnarok.
Known only to a handful is that the Darkhold serves as a conduit to the realm of Chthon. Though that entity is bound under Mount Wundagore, its spirit is free to travel. It is a fundamental nature of the Darkhold to corrupt the spirit and make a person greedy for more power. Prolonged exposure to a First God such as Chthon is more than most mortal minds can sustain without going mad.
Morgan le Fay assembled the book again in 600 CE after millennia of tracking down the missing pages. The magics corrupted her and led the foolhardy sorceress into unleashing Chthon, confident she could control the ancient entity. Instead he nearly destroyed Yggdrasil and attacked the Demiurge, threatening all life in The Nine Realms. Morgana, Merlyn, and the Knights of the Round combined forces with several of the New Gods including a young Thor and Hercules as well as other puissant entities and allies. They ripped Chthon's harmony from his home demesne and imprisoned the ancient god's power beneath Mount Wundagore. The force of the combat disrupted the Darkhold and scattered it through Yggdrasil once more.