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. 
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.