I recently rewatched the anime Demon City Shinjuku. It gave me an idea. Have the biggest city in your setting have a demon gate open in it. The demons take over part of the city, turning it into a literal hell on earth.
For a fantasy city, I'd think the local mages find some way to use the walls around that particular neighborhood to contain the demon outbreak. The demons can't get out, but entering the area is a death sentence.
Maybe the local government uses this as a way to execute criminals. Maybe heroes come from everywhere and attempt to seal the gate. Sneaky demons may actually get over the walls and corrupt the rest of the city. The sorcerer who initially opened the gate may be dead, or he may have become incredibly powerful, but with his death, the gate might close.