The Lady of Pain rules over Sigil with an iron fist. She is sometimes described as a floating, robed, giant-sized woman with a mantle of blades around her expressionless face.
The Lady is said to have the power to control each and every portal in Sigil, opening and barring them at will. The dabus, her servants, maintain the city, forever fixing and patching its streets at her unspoken command. For all her power, she apparently refuses to be worshipped as a goddess, and anyone brave (or foolish) enough to worship her has met a grisly demise in the shadow of her blades.
She has never been heard to speak aloud, only rarely communicating through her dabus.
Her obvious objective is maintaining the balance within Sigil, by throwing defilers and denizens who anger her into one of her magical mazes. Often, she will only interfere when the very balance and stability of Sigil is threatened.
The Lady is an entity of inscrutable motives, and often those who cross her shadow's path, even accidentally, are flayed to death, spontaneously sprout wounds or are teleported to one of her Mazes (an almost inescapable pocket universe in the Ethereal Plane).
Rumor has it that even greater deities have fallen before the Lady. The Shattered Temple in Sigil was a major temple of Aoskar, the god of portals, who attempted to bring the city under his control. After his power had reached a certain point, she killed him with a thought, shattering the grand temple and throwing his priests into the Mazes of her making. The ruined temple eventually became the headquarters of the Athar.
At one point a number of factions attempted to control various parts of Sigil, and fought a terrible war with each other; in the aftermath, the Lady appeared to the leaders of the factions, and communicating through a dabus, ordered the factions to disband under penalty of death. The factions complied without question, and have never risen again. The vast majority of Sigil's denizens dread her apparitions, and avoid mentioning her name aloud for fear of drawing her attention.
Origins and Nature
No one knows how the Lady came to be, what her true purpose or nature is. Hellbound: The Blood War suggests she might be a renegade demon lord or Lord of the Nine. Cage: A Guide to Sigil assures us she has been ruler of Sigil for as long as the city has existed, though she might predate the city's creation.
Die Vecna Die! describes the Lady of Pain as a member of the Ancient Brethren, a group of omnipotent beings who predated the multiverse and know the Language Primeval, a magic greater than the gods. In the module, Vecna became a god and entered Sigil by countering the Lady's ban of all gods from the city for a time. He was able to do this because some of the Language Primeval was confided to him by the Serpent, another Ancient Brethren whose magics were as ancient and powerful as her own. After Vecna was defeated, the Lady used the Language Primeval to repair the damage he did to the multiverse by rewriting it, causing the transition from D&D 2E to 3E.