My fort just succumbed to a Forgotten Beast......composed of vomit. It breathes poisonous vapors so my dwarves could only get a couple wacks in before falling unconscious. One managed to cut off its trunk and another cut off one of its feet. Another fractured its upper body (indicating it had no organs). A war dog managed to fracture its head somehow. But eventually, my entire military fell. It repeatedly kicked and "pushed" my dwarves in the head, killing the vast majority of them instantly. My legendary axedwarf clad in adamantium managed a hit to the upper leg before being insta-killed by the headshot.
This thing is insane. It's obliterated 4 attempts to reclaim with all starting seven having bronze weapons and skill, even though every single part of its body is red or missing (it doesn't have blood). I give up. It's now lost all its feet so it just "pushes" my dwarves to death as they lay unconscious in its vapors. However, even an entire starting seven of axedwarves do nothing. It must be gigantic because they can't sever its head or its legs.
The facepalm moment is this: All cavern entrances are walled up in my fort. There's no way for it to have gotten into my main stairwell! Except....the damn wells in the damn hospital. They fed directly from a first-cavern lake one level below them. It must have crawled up the well. It's the only explanation I can come to, as ridiculous as it is. Those "very few" enemies which can still get up wells just happened to be my very first nigh invincible Forgotten Beast in an otherwise very well laid out, very promising fort...damn.