Doom

Doom was the self-proclaimed "universe's greatest assassin", a member of the Lesser Order of Oberon. She owned a vortex manipulator.

Early life
Doom claimed to have been given the name by her mother, instead of choosing it as some kind of assassin nom de guerre. (WC: Meet Doom - the Universe's Greatest Assassin!)

Hunted by Death
At some point, "someone" arranged for Doom to be hunted throughout time and space by "Death… literal Death". She anticipated that even making a space-time jump every hour using the list of possible coordinates that the Lesser Order of Oberon beamed to her vortex manipulator, she could not outrun the entity for more than twenty-four hours. Her final hope was to find the Doctor before the day ran out. As she started running, she sent out a video S.O.S. from a deserted alien planet, explaining her situation and asking anyone with the means to do so who received the message to contact the Doctor on her behalf. (WC: Meet Doom - the Universe's Greatest Assassin!)

Behind the scenes
Some promotional images of Sooz Kempner as Doom were posted to Doctor Who social media.