Encountered a forgotten beast (feather-covered giant caddisfly), and it spewed ichor all over my entrance to the 2nd cavern layer. I, uh.. think it caused a plague. It started with most of the fighters caught up in the final parts of the battle, they got infections in the legs and feet mostly (so I assume it was those puddles of ichor they were stomping through), and so did their animals. I thought I got everyone set up in the hospital and I thought most were going to survive, but no amount of cleaning with soap and water could stem the infection. They were spreading it through miasma, and so when pets would come in, they would catch it, and so on...
The tantrum spiral started when the first huge wave of pets died. Then hospital patients started dying. For whatever reason, only one dwarf ever got proper treatment (full arm amputation to stem the spread of infection), so everyone just kept piling up in the hospital, spewing miasma and slowly succumbing to infection.
Well, I SAY slowly... I went from 107 dwarfs in year 258 (year of the FB encounter) to FIVE dwarfs in year 260. So much insane shit happened in those two years it almost seems excessive to go over it all. Obviously, plenty of murder and violence. Caged megabeasts being let loose, not enough coffins so bodies piled up around the entrance of the fort, etc... One dwarven child was wandering around the fortress in a foul mood towards the end, using her arms apparently to drag her torso behind her as her legs had completely given over to the rot.
However, those five survivor dwarfs are Very Happy/Ecstatic. Not exactly sure how they managed to remain mentally intact after everyone else went berserk. Plenty of artifact and art installations at major junctions, best quality prepared meals, and everyone having 2x3 smoothed and engraved bedrooms helped, I'm sure.
I think some of them were working and generally staying out of everyone's way even during a massive amount of bad thoughts. I have one legendary axedwarf left in that lot and I think her thought "Took pleasure in the slaughter" was really really accurate, though. She had to put down a bunch of dwarfs and was the first one to come out of the chaos with an ECSTATIC happiness level. I also managed to get a dwarf bone bed out of a fell mood in the very midst of the violence. First fell artifact ever, it's not interesting, doesn't menace with spikes or anything, but it's made out of dwarf bones. Definitely giving that to my axe dwarf.
I've got thousands of barrels of booze, thousands of prepared meals, plenty of coke and ore, and five happy dwarfs. No reason not to continue this fortress, as long as it can attract migrants in the next few years. My post-apocalypse fortress.
(Further, there are no goblins or kobolds, which seems sad, but it's why I was plowing into the caverns in the first place. I needed things for my military to kill. I think I got more than I was prepared for. And unfortunately, none of this helped my FPS much. Preceding the forgotten beast encounter, I had executed an EXTENSIVE rock clean-up operation to pull my FPS back up over 50. It's currently at 68 FPS.)