"The Forgotten Beast Arak has come! A huge beakless swan. It has a long, straight horn and it is slavering. Its light blue feathers are fluffed-out. Beware its poisonous vapors!"
It doesn't matter how many legendary military dwarves you have, complete with exceptional steel armor and named/masterwork/artifact steel weapons, if they have Advanced Rot and Extreme Swelling all over their bodies. And track the syndrome all over the fort.
Lessons learned:
Put in a heck of a lot more work on being able to quarantine the caverns, or parts of the caverns, from the rest of the fort.
At this point, my forts really don't do much with either water or magma. This needs to be fixed. Need to work on building airlocks/decontamination chambers. Would also like to learn how to obsidianize a critter like this one.
Need to learn how to make traps other than the traditional kind (i.e. traps that actually work on trapavoid creatures, like creating a cave-in, or giving a creature a magma bath).
Have more beds in the hospital. Try and locate the hospital somewhere that "forgotten beast extract" won't end up all over the most heavily used corridors in the fort. I don't really need a hospital near the surface; so far, the really nasty FBs come from deep below, and I laugh at goblin sieges at this point. Have a hospital for the cavern levels.
Learn more about how to actually get tiles clean. I read something that suggested that constructing a floor over a contaminated tile will clean that tile. Gotta try that out.
Pay more attention to my medical dwarves in general. Train them more before half the fort is spreading miasma everywhere.
In the case of this FB, I'm tempted to say "use marksdwarves only," but it took 40+ pages of combat text by legendary military dwarves to kill this thing in the first place, and it was a fast-moving flyer. I don't think the marksdwarves would have been able to take it out before it started feasting on my civilians on higher layers of the fort. Plus the vapors had enough range that SOMEONE was going to get infected regardless.