Harbinger of Death - Doctor Who: 2023: Season One

Created by the Pantheon of Discord, Harriett Arbinger was the harbinger of Sutekh, the God of Death.

Sent to Earth to await the return of her liege, she was a sleeper agent stationed at U.N.I.T. Tower, integrating herself into the team. 

With the Doctor away investigating Susan Triad and Sue Tech, she manipulated U.N.I.T. into freeing Sutekh from around the Tardis, ushering in an Empire of Death. With her task complete she turned on her former colleagues, promising to give them Sutekh's gift.