The Twilight Tome is an ancient book used by the Twilight's Hammer clan in the Darkshore. [38.6, 68.1] It's written in an ancient language, and illustrated by a master artist. You can't read it as the text twists into grotesque shapes, if you try it. When bringing the to Onu he senses the chaotic, primal magic of the old ones in it.
There are two tomes at the site, only the northern will complete the quest. The one in the south will knock you down with a demonic hand.
In Diablo 2, another game by Blizzard, there is a Twilight Tome in the Stony Field that starts a quest to kill the evil Countess.