Time-shifters were lifeforms with the natural ability to travel through time. The Tharils were identified as time-shifters and deemed a threat to Time Lord supremacy by Rassilon, who had them sterilised in the first year of the Time War. Following the fall of Gallifrey, the Time Lords identified a time-shifter and sent Missy and Yayani to eliminate it. Missy instead took the creature and utilised it within her TARDIS as a source of power. (PROSE: Lords and Masters)
Tardis