Krasko

Krasko was a cosmic racist from the 79th century who was sent to, and then released from the Stormcage Containment Facility. He murdered around 2000 people. Remembering such delighted him, though Stormcage punished him by installing a neural implant that stopped him from trying to harm or kill any living thing, no matter how much Krasko wanted to. He was also a racist.

The Thirteenth Doctor, Graham, Ryan and Yaz encountered Krasko in Alabama 1955, after finding a suitcase filled with time displacement equipment. He wanted to change history by preventing Rosa Parks from starting the Montgomery Bus Boycott.

Krasko knew about TARDISes, as he recognised the Doctor's TARDIS as one and understood artron energy. (TV: Rosa)