Ademus was one of the most tremendously powerful men in the universe. He had used his powers to try and conquer his home galaxy of Tremanous. He was eventually stopped by the White Guardian who, unable to kill him returned him to his own people for punishment. His people imprisoned him using a semi-sentient machine hidden behind a mirror, that gave substance to his mental images. When he wanted, he could change his environment but his sense of loss and confusion was manifest by the maze of mirrors that lined every corridor leading out from a dark room where his schizophrenia had him sitting at a table with himself but with one who displayed a different side of his personally.
Ademus was briefly visited by the Sixth Doctor and Peri when the TARDIS was drained of its power by Ademus' prison but the Doctor found a way to restore the TARDIS' power and still leave Ademus imprisoned. (PROSE: Hall of Mirrors)