Publisher: Wizards of the Coast, most recently Planescape: Adventures in the Multiverse (2023 boxed set), building on the original 1994 TSR setting.
Centers on Sigil, the City of Doors — a torus-shaped city built around the spire at the center of the Outlands, containing portals to every plane of existence, ruled by the enigmatic and near-omnipotent Lady of Pain. The Outlands itself is the neutral plane bridging the Outer Planes, ringed by gate-towns each aligned toward a different plane’s philosophy.
Reference: dnd.wizards.com