Mine's the result of a bug, but it's still incredibly entertaining. For some reason, a bunch of my dwarves suddenly stopped recognizing a bunch of others as friends, and there was a massive riot. Around 160 dwarves were eventually counted in the total causalities, including all children- that was fairly unfortunate. Now, my dwarves treat immigrants like goblins. But it's not just immigrants- it's all new dwarves.
So, when my newly elected Mayor gave birth to her new baby- she immediately began running in fear, as though it were an invading goblin. Here's the kicker: she was holding the baby. After a long time of fleeing unsuccessfully, she did what any cornered dwarf would do: she sucker punched her own baby and then kept running.
So there she was, running terrified of her own baby, which she was holding, through the crypts and tunnels beneath my city, pausing every now and then to smack it around a little, and spattering the floors and walls with blood here and there. For the latter part of the struggle, the baby was unconscious. Eventually, she must have ripped its throat out or something, because it bled out rather dramatically, and then the Mayor got an unhappy thought at having "lost a child to tragedy."
After reading post and post about everyday-things that occurs in any fortress, this redeemed the thread. One of the funniest things I've read. What actually happened, did they form a new civ that was enemies of the old dwarven civ?
The Fortress died a few years after the bug emerged- though it lasted longer than I expected. Every time new dwarves showed up, they were slaughtered- although traders were treated as politely as ever. Goblins still sieged, and since after the final death toll of the Great Insurrection of Dwellrim (which had a pop of ~180) I only had 11 dwarves left, I was pretty much resigned to the fact that they'd be easily slaughtered during the next siege.
This wasn't the case, however. After this tiny band of dwarves successfully drove of two sieges and four ambush parties with
two casualties (not combat casualties, exactly, but death from thirst after minor injuries- it was a desert fortress- and they were the farmers who were presumably spared during the insurrection because of their dabbling wrestling skills developed fending off goblin thieves, not my soldiers), I un-drafted my marksdwarves (who were already off duty when the sieges came, but still managed to chase away the invaders), and so had
no military dwarves, and no traps besides the cages in the caverns beneath my fortress which I'd used to capture giant bats/batmen/a pair of giant cave spiders (which I was
very disappointed to learn I couldn't breed). Two more ambush parties, one more siege, and
zero casualties later, I decided that my army of wandering giant bats and war dogs was just too formidable- if the dwarves stayed inside, they'd never even make contact with the gobbos before the siege was driven off by my pets.
So I did what any rational person looking for a dramatic end to a dramatic fortress might do: I collapsed the central tower (which contained most of my artifacts, including a wicked obsidian bed) on top of the bulk of the survivors. Still, I had a couple left- so I sent them straight for the next ambush party. One survived, but had a minor arm injury, so it went to bed (can't remember whether it was a he or a she) in its fine green glass apartment, and quietly starved to death while my pets chased away a final ambush.
EDIT: Oh, as to what happened with the bug- I think that they just started seeing immigrants as invaders, and also the bulk of my other dwarves. It seems that any dwarf without military skills was identified as an invader, although some dogs were also killed, and a number of my military dwarves did die- but I
think that that happened mostly as a result of minor injuries sustained in scuffles with their victims sending them to die of thirst in their beds, plus the depression factor, which claimed the lives of ~15 dwarves after the main riot.