Sigil

The place where planes meet.

In Aegis
Appeared as one of the locations for the Fun for Fifteen Door. The party was uninterested, as it "looked like Goatse world".

The party attempted to flee to Sigil during the Goneril invasion of Malvont. A malfunction with the lanterns brought them to the Plane of Shadow instead, but they found a portal (and key) in Shadow Ravinia which led them to Sigil.