A dinosaur-type Forgotten Beast broke into my fort via the aqueduct that carried water from the underground lake into the Magma Sea for obsidian casting. He used his blinding scorching dust along with his sharp fucking teeth to kill all hundred or so dwarves dwelling in the fort. I reclaimed the site, taking with me a steel-armored swordsdorf to try and fight the beast. On initial inspection, it appeared that the beast had fled and the fort proper was completely devoid of life. I sent down 2 miners to check on the (now undiscovered) obsidian caster and prepare space for a new batch. Both were ambushed by the dinosaur and blinded before being eaten inside the dry obsidian mold. I sent my swordsman to try and fight the thing, while I sent my third miner along on a suicide mission to breach the side of the mold, filling it with magma to kill the beast. Both were killed before they could accomplish their tasks, and the beast received only a fractured leg. I decided that one way or another, this thing would die. I forbade all jobs besides mining (which I enabled on all 3 remaining dwarves) and I sent them all down to mine the mold's wall (they only needed to mine one tile). The first dwarf down died before he hit the pickaxe, and the plan completely failed when the beast ascended stairs into the diorite quarry, and then on to the main mineshaft. The shaft passes through maintenance tunnels for the aqueduct, and this is where the beast collided with my bookkeeper (who I made a wrestler and ordered to tackle the beast). There was a useless exploratory shaft which had first discovered the lake (it was just a up/down staircase, with the lake at the bottom). I didn't witness the actions myself, but in the combat log both the beast and the dorf "collided with objects" and when I looked back at them they were both at the bottom of the lake wrestling each other. The dwarf had every bone in his body fractured and he had several appendages removed before he finally died. Meanwhile, the bookkeeper's brave efforts bought the expedition leader enough time to floor over the shaft which the two had fallen down and to wall off all other entrances to the lake. The expedition leader kicked back and sorted through and organized the goods from before the first abandonment until a migrant wave came, and now the fort is back to normal.