The black mace is a mace stronger than the steel mace. It requires an Attack level of 25 to wield, and can be purchased from Flynn's Mace Market in Falador. It can also be found as a fairly rare reward from looting bronze chests in the Shade Catacombs.
As with most maces, this weapon comes with a small prayer bonus. This, combined with its speed, makes it a decent choice for fighting.