Sabaoth

 was a creature that was trapped in a catacomb in Yorkshire. A cult drew-up around them, throughout the ages. It created avatars over time, Giles who accused Nyssa of being a witch, the Judge who sentence Adric to death, and Louie who taunted Leanne. They were able to communicate with Tegan Jovanka through the TARDIS's telepathic circuits and then sent her to 1981. He used Leanne to Manifest. (AUDIO: Ghost Walk)