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 any known deity, she accepts no followers and is known to respond to those who try to worship her with extreme violence
(although strangely, not people who worship her in secret. There is at least one person in Sigil who secretly is a petitioner of The Lady whom she hasn't destroyed). While most Cagers don't like to invoke her name lightly, the phrase "Her Serenity" is commonly used to refer to the Lady of Pain.