Yayani was a Time Lord.
She suffered from Abridgement Syndrome, which meant that she was not able to regenerate at all. She came from the House of Stillhaven and when Rassilon had all the members of the house taken away and experimented on after having their memories wiped. She was only able to remember due to her finding a recall cube that her brother left behind. Following this, she attempted to kill Rassilon but was stopped and given a choice: be executed or work for the General.
Yayani had explosive nanites injected into her hearts that would be triggered if she tried to escape from her captivity.
Following a mission alongside Missy, where they were to stop time experiments conducted in the 28th century, which would reveal that Gallifrey hadn't been destroyed, she was killed by Missy using her TCE which crushed her to the size of a doll. Missy later noted Yayani looked surprised at being killed by the TCE beam. Her shrunken remains were posted back to Gallifrey where the General received it and a note from Missy saying she wasn't going to work for them again. (PROSE: Lords and Masters)