The Bonereaper's Hook

The Bonereaper's Hook unlocks the hidden appearance for Apocalypse, the artifact weapon for Unholy death knights.

Source
This item drops from Stitchwork in Putricide's Secret Lab.

Reaching the lab
Casting Apocalypse (artifact ability) after reaching Artifact Knowledge 6 has a chance to summon an additional ghoul who will speak:


 * say

Upon expiring, this ghoul will drop Professor Putricide's Lost Journal. Reading the journal points you toward Putricide's Secret Lab, via a trap door in Putricide's Laboratory of Alchemical Horrors and Fun in Icecrown Citadel. There you will find Stitchwork; kill him to retrieve the Hook.

You do not need to read the book in order to access the trap door, only summon the special minion. You can't read someone else's book for credit.