Lady Rabia is the leader of the Umbrum. I used to believe that she was the biological mother of King Sombra, but then I learned that she isn't despite the fact that in the comics, she claimed to be his "mother". Though I would say that she considers herself to be the Mother of the Umbrum/Shadow Ponies. Sort of like how Queen Chrysalis is considered to be the Mother of her Changeling army. Lady Rabia and the Umbram were created by Emperor Grogar to take control of the Crystal Empire and its Crystal Ponies to keep them from spreading light and love from the Frozen North.

Rabia (c) MLP:FiM IDW Comics