I had another forgotten beast which caused suffocation. I was building a big magma piston, and had realized I could build it bigger and have more fun if it breached the edges of the caverns. I had just pulled up from a near tantrum spiral caused by crundles causing useless immigrants to dodge to their deaths off the access stairways in the corners of the piston (I was right in that having the piston construction open to all the cavern life caused infinitely much more fun). I only pulled up from that tantrum spiral by hunting down all the crundles with my best axedorf, and plumbing an aquifer through the meeting hall causing a lovely waterfall. One of the junior soldiers was having a really unhappy time, and I remember him tantruming and attacking a legendary, with the legendary patiently blocking and parrying all his attacks until he calmed down (cool guy). On the whole things were looking up.
Then the forgotten beast which had been wandering around decided to come in. I rallied my militia and they quickly put it down, then every single one of them suffocated, except Ol' One Leg who was my most legendary military dwarf (maybe being a cripple provides immunity to syndromes
). Of course losing half a dozen legendary and beloved militia got the tantrum spiral spiraling again (Ol' One Leg was particularly unhappy at losing all his buddies, and at first I locked him up, then I decided if he needed to slaughter some random fish cleaner to feel better about himself it wasn't my place to stop him). While the fort descended into anarchy and the coffins filled I desperately tried to finish the magma piston (it helped that my chief miner had no friends and was pretty happy). Finally I filled the base of the piston up with magma, and my chief miner pulled the lever. Sadly, it was a dud, because some ramps I neglected to remove supported it. In a way it was a fitting end for a failed fortress, all the smiths who the magma was supposed to provide for were dead anyway.
But, suffocation syndromes are annoying, there's not even a slow, gruesome death. Just "hack, wheeze *can't breathe* dead". Need gasmasks.