In Ancient Sumeria, there existed a group of terrible beings called The Dark Ones. Somewhere between demon and man, they created a weapon to use against humankind, a book. Bound in human skin, inked in human blood, contained within the book were incantations and summoning rites, words and equations that went against the grain of reality itself.
The Necronomicon Ex-Mortis. The Book of the Dead.
As well as raising the dead and giving ways to possess the souls and bodies of the living, there are other passages, and other secrets. Namely, the book gives one the ability to travel between realities, opening portals to other realms and other dimensions. (In effect, the Necronomicon behaves similarly to the way a hellmouth sometimes does, but on a smaller scale.) This only makes sense, as the Necronomicon was designed to allow demons easy passageway into the world, so that they may slaughter mankind and take over.
Logs featuring Necronomicon Ex-Mortis: