Lady of Pain

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.

Nobody knows exactly what she is. There are rumors that she's part of a group called the Ancient Brethren (or the Elders), who predate the gods and include such beings as Jazirian, Ahriman and Mok'slyk, also known as the Serpent. When the latter empowered Vecna in his bid to take over the Multiverse and setting up shop in Sigil she was unable to drive him off: while she managed to force Vecna into a stalemate, his presence there would have eventually destroyed the city and with it, the Multiverse, necessitating the party to take him down. This puts her around the same power level as Mok'slyk, and while it gives some answers as to what she is and what her goals are, it raises a lot more questions.

She has two famous unique powers. First, any one who sufficiently pisses her off gets transported to a maze-like demiplane. This is known as getting "mazed". These mazes resemble twisted and infinitely large copies of whatever part of Sigil the victim was in at the time. Victims do not age or need sustenance while mazed and cannot teleport out. The mazes can always (apparently it's an inherent part of their nature) be escaped from but finding an exit can take an eternity. Since the mazes are located in the Ethereal Plane, it's also possible for somebody else to rescue (or kill) the victim, but even finding the right maze is a difficult task, making this highly unlikely.Second, anyone she casts a shadow on has their skin magically flayed from their body. Victims who are killed this way have their souls destroyed so they can never be resurrected.

Crunch

She is left deliberately statless (much like Caine over in another 90s RPG) to prevent powergaming players trying to figure out ways to stomp her, and she's assumed to win any fight by fiat. Combat with the Lady of Pain basically goes like this: The Lady of Pain floats toward you. Her shadow falls over you, and you feel the pain of ten thousand knives flaying the flesh from your bones. You die. The only way to defend against the Lady of Pain's attacks is to grab your character sheet before the DM throws it in the shredder. To further elaborate why messing with the Lady is a terrible idea, the writers created Aoskar, a god tailor-made for worship in Sigil. The moment one of her dabus began to worship him she deleted Aoskar and all of his temples with a snap of her fingers.

Because 5e is made of cocks and 5e's Planescape book is made of AIDS cocks, her "you die" was nerfed. Now if you sufficiently piss her off she "only" sets you to 1HP from raw agonizing pain. If you still refuse to stop poking her after that she just mazes you.

If she knows what's good for her, she stays the fuck away from 15th level 4th edition rogues (how can She cast a shadow on herself?), Pun-Pun, and the Neutronium golem. It's lamentably common for certain kinds of DM to kill her off to show how strong his campaign villain is ("oh mah Gawd, da villain just keeled da lady of pain, he's sooo strong"). Any player faced with such a brand of dumbfuckery should seriously consider rebellion or physical violence.

Some Troll decided to make fun of both Planescape fans and the haters of Dungeons & Dragons 4th Edition by releasing a phony 4e-style "statblock" of the Lady of Pain, which still occasionally pops up on /tg/ when somebody really wants to piss off the Planescape fans, beyond just trolling them about the World Axis. If you actually read the "statblock", it's basically done in the same vein as Cthulhu in most editions of Call of Cthulhu, where Her every is "You're dead, no saves" and Her defensive stats are all some variant of "It don't work, and fuck you for trying".