For her crimes of conspiring to "pervert the course of destiny", Brimo was sentenced to imprisonment in the Eternity Capsule for millions of years. When Nefrin's sun went supernova, the capsule fell into the resulting black hole. Brimo found herself in a blank dimension which could be shaped by her imagination, although the power to do so drained energy from the universe outside it.
Some time later, the Fourth 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 she create the most terrifying thing she could think of. Brimo did so, but realised too late that this was the Eternity Capsule, which formed around her and imprisoned her once again. (COMIC: Doctor Who and the Time Witch)