Short answers, pulled from the story.
Zeb Cook designed the Planescape campaign setting for Dungeons and Dragons. He created the City of Doors known as Sigil in 1994 as a philosophical treatise distinct from medieval European tropes.
The Faction War occurred in 1998 and effectively closed the book on the Planescape product line. Duke Rowan Darkwood instigated the conflict to dethrone the Lady of Pain and rule the city himself.
The third principle of Planescape is called the Center of All. This principle states that wherever a person happens to be is the center of the multiverse.
Planescape Adventures in the Multiverse was released in October 2023 for 5th Edition. This three-volume box set marked the return of the setting after previous editions had discontinued the line.
Dana Knutson drew the Lady of Pain for the Planescape campaign setting. Her distinctive art style became the logo of the entire campaign and was described as resembling the work of Dr. Seuss with a darker edge.