Eons ago Chthon transcribed all arcane knowledge into one book, The Darkhold. Knowing that he needed to leave the Earthly plane to flee the New Gods that were taking over, Chthon created the Darkhold to act as his touchstone to Earth. Chthon can control anyone who uses the spells of the Darkhold and given time he could use it to create a portal to Earth which would allow him to return from his current realm. The Darkhold is also known as the "Book of Sins" because it corrupts the soul of anyone who uses its knowledge.
The Darkhoold Book was created by Gerry Conway and Mike Ploog and was first mentioned in Marvel Spotlight issue 3 and seen one issue later in Marvel Spotlight issue 4 (1972).
Major Story Arcs
The Montesi Formula
As a note, Doctor Strange used the Darkhold to destroy all vampires on Earth and created a spell by which they can never again exist on this plane. However, due to his vast power, Dr. Strange was able to use The Darkhold without forfeiting his own soul.
At one point, a minion of Chthon known as the Dwarf, sent pages of the Darkhold to unsuspected victims, giving them demonic powers. Two members of The Midnight Sons accepted pages of the Darkhold. Morbius, who used a page of the Darkhold to bring his dead girlfriend Martine back to life, but accidently brought the Lilin Parasite as well, and Blade, who was empowered by the Demogorge page and turned into Switchblade, a being set out to destroy all supernatural life. Even Spider-Man was offered a page of the Darkhold, but he refused it. To make sure these pages wouldn't spread any further, the Darkhold Redeemers were formed. The team consited of archeologist of the occult Louise Hastings, Interpol agent Sam Buchanan, and Victoria Montesi who came from a long line of those who protect the world from evil. Alongside them was Louise's grandson Jinx, and Modred the Mystic.
Doctor Doom and Scarlet Witch
Later on, Doctor Doom opened up The Darkhold with the intention of using it. In doing so, he awakened Chthon who had previously been trapped in the Other-Realm. Scarlet Witch knew about this and confronted him, convincing him to work with her to stop Chthon. They then managed to help form the Darkhold Defenders team with the help of Victorious.
Victims of the Darkhold
Blade: Became Switchblade and started killing mystical beings all over. He later on read too much of The Darkhold and was possessed.
Morbius: Resurected his old love Martine but accidently brought forth the Lilin Parasite.
Louise Hastings: Used the book to eliminate Switchblade but lost her soul in the process.
Victoria Montesti: Used the book to save her girlfriend Nash. She got healing power because of this.
Iron Man: Read too much of The Darkhold and was possessed.
Black Bolt: Read too much of The Darkhold and was possessed.
Spider-Man: Read too much of The Darkhold and was possessed.
Wasp: Read too much of The Darkhold and was possessed.