Yeah... I have to agree the pits of hell does not exactly fill me with dread anymore when I find them.. *except I dread the FPS loss until I kill them demons.
I have to admit real spawns of hell and something much more scary would be appropriate... But not just that, some scaling rewards and options would be nice too...
Maybe if we killed demons and received certain artifact armors for doing so...
Maybe instead of one round the demons should pour out for a duration, perhaps forever... So.. over the life of the HFS and your fortress if you somehow manage to hold them at bay (12?) you would have to kill hundreds of them, with some waves being very tough and numerous... and with different counters so not one defense will stop them every time... Like... some demons will have trapvoid, some not... some will be fire immune... some will be almost piercing immune but not bludgeon immune, some the other way around... some will have strong ranged attacks, some not...
Then just throw these after the player until they finally die, cry for mercy, or use adamantium to wall them back in.... *ever seen the writings on the walls in movies, where they warn no mortal to ever break the seal.... Yeah something like this =)
BUT!! for as long as the player wants to RISK having the pits open, they can reap the REWARDS of adamantium, Drops from the demons, perhaps even other things that do not exist in the game yet... Like curses, spells, or even healing methods.. *demons in my mind have always been excellent healers, because they use humans weakness for one and other to get them to make a deal.
Maybe even an option to succumb to the invasion... and in the world your fortress then becomes demon controlled and your dwarves, their little slaves taking on the characteristics of the demon who conquered them...
* demons also need to bring along lackies... Imps, dark elves, deep dwarves, abominations, magma creatures, fire elementals, filth monsters for tentacle demons, ect...