They are a militant group originating from the planet Nemesis in the 30th Century. Using the tremendous evil powers of the Black Crystal, they attacked Crystal Tokyo, pursuing the mighty Silver Crystal and turned all of the people into suspended animation, as well as the Queen herself. They eventually travelled back to 20th Century, pursuing Chibiusa Tsukino.
They started countless conflicts and committed mass murder. Eventually they made Nemesis their headquarters and then started a full-fledged attack against the Earth.
The members of the Black Moon Clan have black upside-down crescent moons on their foreheads. And have diamond-shaped earrings made out of the evil Black Crystal that allow them to teleport over large distances in a flash of darkness.
The Black Moon Clan live in a castle on Nemesis.