Harriet Arbinger - Doctor Who: 2023: Season One

Believing herself to be a U.N.I.T cryptanalyst, Harriet Arbinger is a sleeper agent for the God of Death, Sutekh, helping bring him into the world and usher in an Empire of Death across the Universe. 

Unaware of her original mission or purpose, she aided the Doctor and his allies in opening a Time Window to Christmas Eve 2004. Making contact with the God of Death after he killed Colonel Winston Lemuel Chidozie, she was now activated. 

No longer Harriet Arbinger, merely a puppet and Harbinger of the God of Gods, she manipulated the other scientists into releasing Sutekh into the world, while the Doctor was away from UNIT Tower investigating Susan Triad and Triad Technology (nee Sue Tech).