Mestizer

Mestizer was a shapeshifter who had a long history with the Doctor.

Mestizer was a time sensitive. Amongst her talents, she was a hypnotist, a mystic, could read minds, and was described as a "powerful creature with many followers." (PROSE: The Tunnel at the End of the Light)

At the bay in London, Mestizer met the Doctor. shortly afterwards, someone shot him in the back and he regenerated into a new incarnation. (PROSE: The Cabinet of Light)

In 1950, Mestizer commanded the Subterraneans to kill several individuals across London, setting the Demon King free. Mestizer was killed by the Demon King as it broke free from the London Underground; but after Honoré Lechasseur and Emily Blandish changed history to stop the Demon King, the events leading to Mestizer's death did not occur. (PROSE: The Tunnel at the End of the Light)