Genned a new world in 40.06, and proceeded to embark in a conflux of four terrifying swamps. Having embarked in terrifying biomes before, I wan't too worried. I'd had clouds that made the giant mosquito swarms vomit all over the place, cyan exsanguinating ooze, purple goo that didn't do anything at all, and rains of delicious elf blood before. I knew the biomes could be bad, but it seemed much likelier that I'd get a random syndrome cloud that didn't do much. But no, this time the RNG decided it wasn't screwing around.
I wasted some time building a palisade until I discovered that there were Giant Sparrow corpses running about, and I'd have to build a roof as well. I tunneled down, discovered the aquifer, and started work on the plug. When the cloud of nefarious ash decided to roll over my camp, I sent everyone to emergency burrows, and things appeared to be fine.
Nope.
You see, animals don't respect burrows. If I'd just made a butcher shop and killed all my livestock, I could have survived this, but I didn't, and they were still alive for the ash to touch.
I don't know what the ash does, even now. Whatever it touched died instantly. Not twitch-reflexes-required-to-see-the-suffering fast. If the ash was visible in the square, whatever was in the square was already dead. Believe me, I tried to catch it in the act, and no matter how quickly I escaped and reentered view creature mode, it didn't work.
And worse? Whatever died reanimated in seconds. The ash took out a horse. Before the cloud had moved another two tiles, that horse corpse was assaulting a donkey who had escaped the cloud, and almost immediately the now-dead donkey went after the dogs...
Now, oddly enough, the various undead livestock were content to murder one dwarf apiece, and then stand about placidly. But I panicked and had my dwarves seal up the tunnel to survive. Except they refused to do it until I let them out of the warrens. And then most of them proceeded to seal themselves outside while I watched the miner.
One was killed by an ash cloud while fishing. One was kicked to death by the horse corpse. One by the donkey. One was killed by a dog corpse, and his corpse proceeded to stand back up and kill his friend. The woodcutter, embarrassingly, had his skull torn out by a cat. Only the miner was left, and I figured if he could just finish and drop the plug, there was a chance he could hole up long enough for caravans and migrants to arrive while he excavated some stone for use in real dwarven architecture.
Except then he went on break. And an ash cloud spawned right over the plug pit (I know, exposing it to the surface was dumb, but it was a shallow aquifer and I needed all the z-levels I could get for the plug.) and the last thing the miner said as the green ash of death barreled down towards him was "This looks like a good time to take a nap!", promptly falling asleep. Then the ash touched him and the fortress crumbled to its end.