Shatterbone, Acolyte of Torment is a mini-boss Super Unique Dark Berserker guarding the entrance to Alcarnus in Diablo III. It's hard to ambush him, as he waits to jump out of the portal at the very narrow bridge.
He is much larger and faster than a normal Berserker, and his attacks are greatly empowered (specially the charged slam attack). He is usually accompanied by numerous other Coven monsters (Cultists, Berserkers and Hellions), but has no affixes as of Patch 2.0 (previously had Frozen).
Killing him is not required for the City of Blood quest, but the fight itself is hardly avoidable.