That would have to be the first and second end to my themed game: Scoutsbridge. I specifically put in a criteria that I could not assign labor to unskilled dwarves (sorta, there were a few caveats).
The embarkation place had an aquifer and, due to lack of the appropriate labor to cut down trees, I was unable to penetrate it because I had no wood or stone to make screw-pump components. While waiting for a good immigration wave, I had a mason start a strange mood that required stone and gems, neither of which I had. Moody dwarf goes berserk and starts killing people. Eventually he's taken down by a peacock. Unfortunately, because the aquifer prevented any stone collection, the mason was friends with absolutely everyone. Tantrum spiral ensues.
During the brawl, that same peacock kills four dwarves before eventually succumbing to suffocation from damaged lungs.
Anyway, tantrum spiral goes unchecked through the population until the last dwarf inevitably bleeds to death from his wounds.
Decide to try to reclaim. New embark, same rules.
Turns out that, once again, no wood cutter, and the migration waves don't bring any. This turns out to be a problem since there are ghosts and no way to put them to rest. The ghosts start out pretty benign, just hanging out in the halls and being all spooky. Eventually, they start getting angry and vilolent. Around the time the ghosts have killed the fifth dwarf, I begin to realize that the situation may not be salvageable. The sixth dwarf killed happens to be a popular mason, which once again kicks off a tantrum spiral. First time I'd ever had a fortress killed by ghosts.
Luckily, third reclaim effort went a lot better.