A powerful being mentally tortures an orphaned girl through isolation and deadly "games" with dangerous machinery. Later, she begins hearing the voice of a mental illness, and through its advice manages to behead the orchestrator.
!The powerful being/orchestrator is GLaDOS, the orphan is of course Chell, and the mental illness is Wheatley. I know that last one is a bit of a stretch, but he technically is if you consider that attaching him to GLaDOS to inhibit her "brain" functions is what he was designed for.!<