The Blood Cellar is a small dungeon in the Black Canyon Mines in Act II of Diablo III.
It contains an unmarked event: a dozen of Fallen Hounds feasting on corpses. They will not attack you first, but if you kill them all, a survivor will climb from beneath the rotten bodies, telling you that cultists threw him here to be a plaything for the hounds. He will reward you with a magic item and then leaves.