I think this thread should be dedicated to making demons more vile! I remember the first time I encountered demons... I wasn't very good at DF at the time at my fortress was little more than a pathetic set of small rooms, basic workshops, and a farm, I hadn't gone into the game more than 3-4 months and had only just set up a mediocre trap system. My fortress wasn't even in a mountain! My dwarves were clearly insane, no sane dwarf would agree to go on the expedition so they could live in a filthy hole in the ground. Still, I had ambitious plans. I was going to create the grandest catacomb that the world had ever seen! ...It was only natural, however that this catacomb should be placed rather deep down. Very deep down. I was just carving out 4-way intersection, which was 5 tiles wide, for my tombs, and all of a sudden... "You have broken into a peculiar chamber through the base of the mountain." I knew of the demons, and as you can probably imagine I was a bit disturbed by this seeing as my dwarves wouldn't stand a chance against an angry camel, let alone a horde of vile demons. I immediately ordered my miner to seal the chamber ASAP, but only 5-10 seconds later, "Horrors! Demons in the deep!" My miner was only given a very brief period of time to loosen his bowels before he was incinerated by a spirit of fire. I slapped the space bar to give me some time to think, I was so screwed it was mind boggling. Regardless, I couldn't stand the idea of my dwarves running in terror, the demons would probably get a kick out of that. I recruited them all and sent them to their deaths. I wanted them to die with honor. ...This was difficult, because even if I HAD weapons and armor to accommodate them with they probably wouldn't reach it in time. You see, the very center of my tiny fortress lead 20 or so levels down to the catacombs, the demons didn't have to go very far. So they marched to their deaths, protected by nothing but their highly flammable pig tail garments and nothing to kill them with save their own bare hands. Their bravery cannot be faulted. Only their instinct of self preservation. They didn't even reach the stairs before a couple of them were incinerated and they started to seriously reconsider their display. Despite their efforts to survive, in the end they were all KFD. While there a dwarf or so left I went to the surface above my literal hellhole and saw smoke billowing out of it. The surrounding forest was catching on fire. And then the game crashed. Probably because Armok was crying.
I'm not even sure whether or not there were mountains in the area AT ALL, but this means that demon pits don't have to be directly under a mountain. Unless it was a big. It's experiences like this that make me want more demon variety. I have a few suggestions:
"Demonized fortress": Returning to your fort would reveal that the demonic presence had spread through your fortress and the "infected" areas now appear similar to the chambers, minus the glowing pits. Pools of magma would be spread throughout your fortress and some constructions might have been ransacked. Rather than being scattered all over the place, all your fortresses items would remain where they were. Some of the dwarves from the fortress before the invasion might have been spared by the demons so they could be tortured. The demons would stay in the pits and all items from the old fortress would be forbidden by default, so you'd be fine as long as you didn't disturb the chambers, giving you time to prepare an attack.
More torture: Demon torture victims would have a special happiness level called "in agony" and would constantly scream, having no effect other than disturbing your dwarves. In the thoughts and preferences screen there would be a list of the horrid things that happened to them.