Krasko
Krasko was a racist murderer from the 79th century[disputed statement] 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 restricter into his brain, which stopped him from inflicting harm or kill any living thing, no matter how much Krasko wanted to.
Krasko's racism led him to believe that "things started to go wrong" with Rosa Parks' victory. Using his vortex manipulator, he travelled back in time to change history.
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.
Krasko's plot was averted by the Doctor and her companions who managed to ensure Rosa began the boycott. Krasko's vortex manipulator was destroyed by the Doctor, leaving him unable to time travel. In addition, Ryan Sinclair used Krasko's own time displacement weapon to send Krasko as far into the past as he could set the device for, forever stranding Krasko in the distant past. (TV: Rosa)
