Sigil is the city at the center of the multiverse, and is inhabited by all sorts of strange creatures. The city is inside of what is essentially a hollow donut, with a gravity that allows for people to walk all the way up and around without falling.
Strix lived in Sigil as a member of the Dustmen, one of the many factions that exist in Sigil. Shemeshka the Marauder lives here as well.
The group were pulled into Sigil by a mercykiller who was trying to abduct Strix. The group killed the mercykiller and left the city immediately. [1]
Shemeshka banished Diath to Sigil to keep him safe from an imminent explosion. Diath found himself at a party in a building with a strange clock that moved in bizarre ways. [2]