So, I had a pretty solid fortress with a very strong militia, multiple legendary warriors, stout and strong. Very few casualties over the years, mostly due to civilians being reckless in the caverns. I'd already killed three or four forgotten beasts, so when I was informed that there was a three eyed, trunked, essence-breathing periwinkle crab in cavern 2, I didn't worry overmuch. I sent my army to the usual ambush spot and watched its movements.
Being a crab, it found a nearby pond and stayed there.
I didn't want to let my troops starve, and didn't want to send them after it, so I sent them back to training and figured I'd check on Mr Elephant Crab every now and then.
And once I checked, and it was already in cavern 1 (they're linked by a tunnel) and barreling for the stairs to the surface.*
I didn't have time, I thought, to check if anyone else was below, and so I sent my troops to the ambush site, checked again, and the crab was already past it! Quickly sent them to the outer court and watched, and there it came, immediately blasting dust everywhere.
Now, it seemed that this crab wasn't heavy enough to deal with the recoil, so with every breath it slammed itself into nearby walls and floors, and likewise the first responders, who were of course the finest fighting dwarves of Worksack. Still, nobody seemed to be getting seriously hurt until the blasted thing finally slammed its head into the ground and died abruptly.
I sighed a sigh of relief, and waited for the dwarves to get up. They didn't. Oh, they must be injured, but what is that blue thingie flashing. What's this? Paralyzed? Suffocating! Ahhhh!
There went a third of my army, including our great spearmaster commander, dead to a badly engineered or suicidal periwinkle crab.
I just hope whatever-it-is doesn't spread, but so far it doesn't seem to be. The army is replaced, but I am not so confident anymore in its ability to fend off the green menace.
* My fortress has an inner and outer court, and the stairwell comes out onto the outer court.