The Cylox were a massively powerful psychic species.
Two of their members, Tko-Ma and Lai-Ma, were imprisoned in the 15th century on an astral plane for misuse of their powers, including manipulating and destroying entire galaxies. However, each was able to establish a connection with an Earth psychic, and spent centuries trying to free themselves. They were stopped and re-imprisoned by the Sixth Doctor.
The Doctor speculated that Tko-Ma and Lai-Ma were actually the equivalent of children. Sebastian Malvern told him that things like Gallifrey, Rassilon, the Pythia, Zagreus and Omega were insignicant compared to the Cylox. On a psychic power scale where the Doctor was 6, Trey Korte a 7 and Malvern a 50, Tko-Ma and Lai-Ma were level 1000. (PROSE: Instruments of Darkness)