It wouldn't take much to make dwarf kind fear Hell and forgotten beasts again.
First, slightly tweak the IA behavior of demons and other intelligent building destroyers so that they prioritize pathing to and killing dwarves above all other things, only stopping to utilize their building destroyer abilities on doors and other constructions that are blocking their path to would be victims.
Second, add a building destroyer 3 tag to the game and have it appear as a potential random ability that procedurally generated entities such as demons and FBs could be assigned during their creation. The level three building destroyer tag would simply allow the creature to destroy any construction in the game including artifacts, walls, hatches, bridges, fortifications etc.
Third, add an intangible/phantasmal tag that this same group of procedurally generated supernatural creatures can also be randomly assigned. This ability would let the entity phase through any solid material with the sole except adamantine. Thus they could path through stone, soil, metal etc. as if it wasn't there. It would also naturally make the creature immune to cave ins and other physical threats while it is intangible. Phantasmal entities would have to make themselves tangible in order to attack opponents or destroy objects at which point they would naturally become vulnerable to attack and subject to all the normal rules of dwarven physics while maintaining their physical form. This would be a particularly dangerous ability and thus would need to be somewhat rare, kind of like how deadly dust is currently.
I also agree that Hell's design itself could definitely use a bit more flavor and many of the suggestions which have already been made in this thread are excellent.