The Rock Demon is a Boss found in the Abandoned Mine's and has a small percent chance of spawning when a player kills a Rock fiend.
At various points during the fight, the Rock Demon will begin soaking damage. All attacks done to him at this point will be returned as one hit back to the player when the soaking ends, potentially dealing thousands of damage to a single player. The boss also can spawn minions ( Rock fiends ) to help it fight.
This can be obtained as an extra item roll rarely from all monsters. They will only drop if the monster is near your level (bosses not included)