Brimo

Brimo was an arch-criminal from the planet Nefrin. For her crimes, she was sentenced to imprisonment in an eternity capsule. When Nefrin's sun went supernova, the capsule fell into the resulting black hole, and Brimo found herself in a dimension which could be shaped by her imagination, but the power to do so drained energy from the universe outside it.

Some time later, the Doctor and Sharon were pulled through a time rift into Brimo's dimension, where she attacked them. Since the Doctor's TARDIS was preventing energy from entering the dimension, Brimo began losing her power. When she only had enough left for one last attack, the Doctor suggested that she create the most terrifying thing she could think of. Brimo did so, but realized too late that this was the eternity capsule, which formed around her and imprisoned her once again. (DWM: The Time Witch)