An ant queen just destroyed my entire military.
I knew there were bad things in the cavern layers. However, I expected the antmen to be relatively pissant enemies. Three squads went in to destroy a small group of antmen. It looked like they'd won; all that survived the first two seconds was a single ant queen with every bodypart showing broken, cut open or smashed.
I figured it would bleed to death eventually. I was wrong. I thought it was a different ant when it came back a second later to gnaw on a soldier with a broken foot because now none of it's limbs were broken. A single hammerdwarf decided to break most of them again, but I ordered all three squads back down the stairs to look for any anthills that were producing these things.
I was wrong. It was the same ant queen I'd let go before, not a new ant. A swarm of dwarves chased her around the caverns, breaking and cutting open bodypart after bodypart - but all the damage just healed up. Her description showed as an unusually long and rapidly expanding list of scars and healed injuries on her black chitin.
Most of my military hit Elite status before dying or becoming too tired to continue fighting. However, they reached their limits eventually and with the regenerated Queen below and a goblin siege at my gates I decided that the fortress was lost.
.... excuse me, but what the heck was this? The wiki doesn't mention that they regenerate so fast that 30 dwarves (mostly silver warhammers and steel spears, plus an axe and some crossbows) pounding on them at once for a few minutes won't do them any lasting harm. Did I experience a weird bug (pun intended), or are all queens this incredibly resilient? If they are all invincible I am going to go back and have fun inventing ways to abuse this property as a training aid and goblin torture technique.