Mind control
Mind control is a common occurence in science fiction and fantasy. The methods vary, including psionics, drugs, parasitic creatures, technological devices, and magic. Regardless of method, mind control allows one person -- typically a villain -- to control the thoughts and/or actions of another.
The degree of control also varies, ranging from subtley influencing a person's behavior to permanently altering their personality. In most stories, the degree of control is somewhere in between, forcing the victim to take actions he or she would normally never do without actually making them "willing" participants in the act. Victims are often able to resist the control to some degree, even "snapping out of it" if the controller attempts to force them into a particularly despicable act.