MarkB
Legend
One option is to give the demons a reason to stay. Maybe a group of the more powerful ones in the buried centre of the city were able to carve out a protective bubble for themselves in the lava, along with some innocent civilians, and in the intervening years have made it their own personal little hell, magically maintaining the citizens, torturing them by repeatedly letting them burn almost to death and then restoring them.
PCs who explore the city think they hear tortured wails and assume it's either their imagination or some lingering ghosts, but if they follow the sounds to their source, they can break into the bubble and attempt to free the trapped people, even if only to let them finally die and pass on to the afterlife.
PCs who explore the city think they hear tortured wails and assume it's either their imagination or some lingering ghosts, but if they follow the sounds to their source, they can break into the bubble and attempt to free the trapped people, even if only to let them finally die and pass on to the afterlife.