Twilight Canticle

The Twilight Canticle is a book written by Cho'gall during the First War to codify teachings about the Void. As the ogre mage attuned himself to the Old Gods, he branded the prophecies of the Twilight's Hammer onto the skin of his Pale orc followers before cutting off their flesh to form the pages of the book. The Twilight Canticle would become a source of power and motivation for the followers of the Old Gods for decades.

Speculation

 * The book may be the reason why the Twilight's Hammer eventually became a multicultural sect instead of a mere orcish clan, its teachings bringing humans, dwarves, elves and other races to the cult.
 * The Twilight's Hammer occupied Northshire Abbey during the Second War; it's possible that they left the Canticle behind in Northshire, leading to it eventually ending up in the hands of impressionable members of the Stormwind branch of the Church of the Holy Light, like Bishop Farthing and presumably the Black Bishop.