The Cypher of Broken Bone

The Cypher of Broken Bone unlocks Bonebreaker appearance for The Kingslayers artifact.

Source
After reaching Artifact Knowledge 5, it has a chance to drop from from rares in the Underbelly, part of the Sewers of Dalaran.
 * Thieving Scoundrel
 * Splint
 * Mana Seeper
 * Screek
 * Segacedi
 * Crazed Mage
 * Xullorax