Krasko

Krasko was a murderer from the 51st Century who was sent to, 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 murder anyone, no matter how much Krasko wanted to.

The Thirteenth Doctor, Graham, Ryan, and Yaz encountered Krako 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)