Darkhold
The Darkhold is a volume of parchments transcribed eons ago by the Elder God Chthon containing all of the arcane knowledge of the time. Chthon learned that new gods were supplanting the old on Earth, and in order to protect himself he decided to flee the Earthly dimension. Before doing so he created the Darkhold to serve as his touchtone with the physical plane, as an indestructible medium through which he could manipulate others, and as a talisman that could be turned into a dimensional nexus for his eventual return.
The volume was first discovered by human sorcerers of pre-Cataclysmic Atlantis who managed to remove it from the doomed island-city before it sank. They founded a cult called the Darkholders and used the spells contained in the tome to create vampires to do their bidding. The book passed through a succession of hands through the next millena, including Babylonian savants, Egyptian priests, and Hebrew scholars. Whoever employed the knowledge contained within did so at the cost of the corruption of his life essence(soul). This earned the Darkhold the name "Book of Sins".
The parchments were eventually bound into book form in the Sixth Century. It lasted in this form for many centuries before an Irish monk scattered the indestructible pages throughout Europe in a the hope of removing the temptation once and for all. In the 12th Century the pages were mystically reassembled by a Spanish monk and later, in Transylvania, a scholar named Baron Russoff rebound the parchments in book form, adding blank pages in the back to serve as his diary of occult experiences. The possession of the Darkhold triggered his hereditary tendency toward lycanthropy and turned him into a werewolf.
Eventually the tome found its way into the hands of Dracula, Lord of the Vampires, who was looking for a means to restore his flagging powers. Finally, realizing that within the Darkhold lay the Montesi Formula, the spell to destroy all vampires, Doctor Strange sought the book out, fought with Dracula and managed to use the Darkhold to create a spell by which vampires could no longer exist on Earth. Even a vampire who was in another dimension or world and returned to Earth is destroyed. This is why there are no longer any vampires on the Marvel Universe's Earth.
Due to his vast power, Doctor Strange was the only person to ever use the Darkhold without forfeiting his life essence(soul) to Chthon. Others who have been corrupted by the Darkhold include the 6th Century British sorceress Morgan Le Fey and a disciple level sorcerer named Modred the Mystic. Many, many others have fallen to its evil.
Any character not a Sorcerer Supreme who even touches the tome immediately turns evil and serves the daemon Chthon. That character becomes a non-player character then, keeping his ability to perform the spells that he knows, but learning many vile and evil spells from the Darkhold. The Book of the Vishanti holds a spell that will release a character from the Darkhold's influence, but it will be difficult to find.
All characters who are magically inclined have heard of or instinctively feel the evil of the Darkhold so any desire to touch the book must be made with their complete free will. A character who tries to psionically or mystically force them into touching the book is in for a surprise. The unwilling character acts as a conduit for the Darkhold's corruption and the manipulating character is, instead, the receiver of the curse. A character manipulated in such a way will remain his own person if he immediately drops the tome upon release by the stunned manipulator.
The power of this book is so great it can provide power to its followers in other dimensions. Chthon has tried to return through its use many times in the past, most recently using as a host body the Scarlet Witch. The Darkholders still exist as a cult, and are dedicated to finding the volume and using it to gain ultimate power. It is presently under spell, lock, and key at Doctor Strange's mansion.