Lady of Pain
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The Lady of Pain is one of the primary figures of the Planescape campaign setting. She is an immensely powerful being who protects the city of Sigil from outside forces. Despite being at least as powerful as many deities, she accepts no followers, and is known to respond to those who try to worship her with extreme violence.
She is left deliberately statless to prevent powergaming players trying to fight her or actually coming up with ways to defeat her in combat, and is automatically assumed to win whatever fight she gets into. The only way to defend your character from the Lady of Pain's attacks is to grab your character sheet before the DM manages to tear it up.
If she knows what's good for her, she stays the fuck away from 15th level 4th edition rogues.