Well, things typical to dwarf fortress happened.
My digging down paid off and I found and adamantium vein. I began hauling this up the 108 z levels to be stored ready for my Legendary Armorsmith to make something terrifying from. Just as I was finishing this, Forgotten Beast number 7 turned up. No problem, I thought. My army of 30 will dispatch him as quickly as they did the last 6, given they are nicely kitted out and mostly hammerlords and axelords...
So the fight begins, without much of an issue he is killed with one casualty, a recruit who got pulled limb from limb. Then strange graphics flashing on the tiles... oh its fire. Everywhere. The beast seemed to have burst into flames, either that or it breathed a shit load of fire just as it died. One by one, over the space of 6 or 7 seconds, all 30 of my dwarves died. Some seem to have been poisoned, some from blood loss, others Im guessing from the raging shit storm of a fire.
My disappointment wasn't to stop there. The 20 or so War dogs that accompany my 3 squads are mostly alive and heading back up into my fortress. Fair enough I think. Until they too start dying from infection, just as they arrive at the meeting/food hall. Now, my food halls always seem to consist of a lot of dwarves and a ton of their little pets, rabbits, cats, dogs, goats etc. I allow them that small privilege. As the war dogs drop from infection, miasma clouds burst and spread rapidly. Other animals start dying from infection. Oh god.
Prior to this, Id linked my danger room up incorrectly, having been distracted. Id linked it to my bridge lever as well as my spear spikes. Id pulled it and not thought anything about it, about 5 minutes before the beast arrived. So, a further 5 levels up my fortress bridges had been raised, making the outside world a no go zone, including my corpse areas.
By the time Id figured out why no corpses were being removed, my population was down from 127 to 83, and most of the animals were dead.
Never again will I consider myself "set up and ready for anything". The hatches to the caverns are now sealed. The fort is clean, most of my dwarves are unhappy, mass production of coffins is taking place, plus slabs for those bodies still stuck in the caverns.
As a final point of misery, my golden artifact greaves, my artifact jaguar leather quiver, and my artifact bone shield are all trapped down in the firestorm caverns. I'll have to send down a party to get them, although I don't hold out my hope of them returning. There's blind ogres, troglodytes, another forgotten beast with poison, a heard of naked mole dogs and the ghosts of about 30 angry dwarves to get through.
I might flood the whole place.