Sigil is a city that exists outside the planes and connects to all of them. Known as the City of Doors it has portals to various places in the planes. It is ran by the enigmatic Lady of Pain. She does not take any active role in governing and leaves the city to it’s own devices but she has unimaginable power and it is enough that she keeps the gods themselves from being able to enter her realm. Also exhibiting power to kill with a look or return the dead to life. Nothing is really known of her but she destroys those who take to worshiping of her.
Sigil itself is shaped like a torus with the city located on the inside walls. Because of its connection to the planes creatures from all locations can be found here. Demons walk next to angels, and mortals by goods from fey and elementals alike. The City is ran mostly by different factions but only as The Lady allows. As long as it doesn’t interrupt her unknowing plans it is allowed to go on.