My oceanside desert volcano fort's nemesis was this damned fireman who had a talent for picking off the most valuable dwarves at the worst times. First time he came out after one of my miners was drafted to kill 4 fire imps while clearing the area around the volcano's mouth, he succeeded but broke a leg, so the fireman slaughtered him easily then ran before my other two miners could come and gang him.
He was content to hang out down there for the longest time, I usually clear out magma pipes before setting up significant industry but I needed metal so I risked it anyways, set up a forge and a few smelters with my two swordsdwarves on guard, they both left to get a drink at the exact same time so the demon decided to come out and burn my metalworker to death. He got a name after this kill. Later he came out and killed someone who was cutting down cacti for wood, though he took a wound to the leg for it.
Then came the true rampage, I had finally finished the room around the volcano's mouth, even taking care to specifically wall off the bar and armor/weapon stockpiles and put stone doors on them so the bins wouldn't get burnt in a rampage. I also put the barracks inside the room so there'd always be dwarves right there ready to fight the fiend. He showed himself again at the worst time, my 6 dwarf military was all sleeping or away. He ran right for the barracks and let loose a gout of flame, which burned everyone in there alive in their beds.
He then proceeded to leave and start terrifying the metalworkers, who lingered around there regardless of me making the entire metalworking area a restricted zone. He chased one right into the damned bar stockpile. He attacked with fire and caught all of the wooden bins on fire, including all of the coal and lignite I'd traded for for steel production. Oh, also the dwarf he burned to death was a legendary armorsmith who was BFF with all of the military because I carved all of the rooms and a dining hall for the metalworkers near the forge room, where they all mingled frequently due to unfortunate similar lazy personalities.
All of the forge staff and military immediately became unhappy. But the fireman hadn't yet been killed. I'd locked up the forge and sent the three dwarves I had in to end it once and for all. But unfortunately one of them took a mangled arm trying to wrestle it and once again it escaped into the depths.
The wounded arm combined with dead friends quickly drove that dwarf insane. The minute he recovered from the wound he went berserk and threw a furnace operator across the room, where he landed in a magma channel which was unfortunate because he was wearing an artifact silk tunic which then made my legendary clothier miserable. One of the two remaining veterans of the skirmish in the magma room put him down, then immediately became miserable because it was his best friend. Before he even got back to the barracks he became stricken with melancholy and threw himself in the magma pipe, which drove my last veteran into stark raving madness, where he eventually died by throwing himself in the magma pipe.
Then the clothier also went berserk and considering I was cross training him with pumps prior to him making his artifact he was tougher than average. He managed to kill a war dog I assigned to him then killed two children and a cat before getting killed by another war dog which sustained fatal injuries and shortly after passed out on a trap and died. Oh, also both kids and the cat belonged to my third metalsmith who went berserk shortly thereafter and murdered two furnace operators before being captured by a cage trap. As my mechanic (one of my starting dwarves) came up to re-arm the trap the fiend appeared yet again, scorching him to death then running into the pit before my newly drafted recruits could respond.
Oh, did I tell you that said mechanic was also my mayor, best miner, and best friend to all of my other remaining starting dwarves? I'll spare you that segment of the tantrum spiral. Needless to say, by the time it was over I was down from a peak of 54 dwarves to 28 dwarves, and the only starting dwarf remaining was my farmer who 'doesn't care about anything anymore'.
Finally I had resolved to find a final solution. I channeled the magma into a square then devoted half of my fort to masonry and the other half to military, with a couple of support farmers/brewers keeping them fed and drunk. I planned to scrap all of my old furnaces and forges then floor over the entire damned magma pipe with green glass blocks, then go back and remove tiles to place new furnaces. I never found out if this would have worked or not, because when my military decided to go eat and sleep the fiend emerged again and burnt most of my masons to death. I recruited the masons who still lived in an act of desperation and one mason, Urist Mengdomas, lept upon the fiery beast and got it in a brutal headlock (and in my imagination he did a lot of sweet wrestling moves too) and broke the damned thing's head off. He was rewarded by mortal wounds to most of his organs and a bruised spine, but he finally killed the fiend.
By this point it was only a matter of time until the spirals killed my fortress for good so I made everyone engravers and masons and put all of the wealth I possibly could into his tomb. He had an emerald encrusted golden coffin and gem encrusted obsidian statues alongside legendary engravings all around his tomb. Shortly after I finished the spirals finally finished me as my last former military dwarf punched my berserk legendary engraver's head off, then threw himself into the magma pipe.