The off-hand black warhammer is the off-hand equivalent to the Black warhammer. It requires 25 Attack to wield. It can be obtained from chests that require Steel keys in the Shades of Mort'ton minigame.
Before an update on 17 June2013, all off-hand black equipment, with the exception of claws, was unobtainable and could only be seen by searching for it on the Grand Exchange.