Brain-worm

 was able to eat peoples mind. It followed the telepaths, Tessa Waylund, Gorden Latch and Jane Thirgood as well as Charlotte Pollard around The Cube and captured the Eighth Doctor and C'rizz. It took part of the Doctor's memory. No-one had physically seen it. It was created by a faulty experiment, when trying to remove Tessa Waylund's psi powers. It originally resided in Tessa's mind, then transferred to The Doctor's, and then into Latch's worm. (AUDIO: Something Inside)