Mercy Hartigan, or simply Miss Hartigan is the main villain in Christmas special episode "The Next Doctor" (2008). She is the willing accomplice of the Cybermen invasion of London in 1851; she resents her patriarchal oppression by man as a woman in Victorian England and seeks to empower herself through any means.
The Cybermen betray her when they decide that they wish to make her their 'Cyber King' in a process which is supposed to remove her emotions and upgrade her, but it doesn't work due to her brilliant mind having overcome the programming. Miss Hartigan is given access to the full bounty of knowledge of the universe and in the colossal 'Cyber King' dreadnought-class assault droid, she seeks to make London bow at her feet as its maniacal dictator who combines cold Cyberman efficiency, knowledge and logic with human passion and determination. The Doctor offers Miss Hartigan the opportunity to relocate herself to another world which she can reign over harmlessly, but when she refuses he cuts her connection to the Cybermen and allows her to see what she has become rationally. In her moment of lucidity, she becomes insane and then dies, as do all the Cybermen who are connected to her.