I'm still a pretty big noob in Dwarf Fortress, the longest one of my fortresses has lasted is 5 years, and by then my ten remaining dwarves were too upset about everyone dying after two forgotten beast attacks. The first one was a giant insectish creature, named Tunmos, that had ice dust. I sent my ragtag military to attack it. They got some good hits on his legs but were blasted into a wall and started rotting and swelling. All my dwarves rushed down to get their stuff too fast for me to realize and they got blasted around and torn apart by the beasts large mandibles. My remaining 40 or so dwarves rushed the survivors into the "hospital" (small room with 4 beds), somehow sneaking past the beast. I checked on the hospital to find that my rotting, swelling dwarves were piled into the corners of the room on top of eachother, including 2 of my military members who somehow survived. I checked the unit menu to see my dwarf count, but I saw that the other creatures section had one less creature than before, I checked it and saw that Tunmos had died! I immediately checked the combat log and apparently the corpses of the dwarves Tunmos killed fell down on him, breaking his bones and the last one smashing his head. The way I imagine it, Tunmos tried to hit a dwarf with his ice dust, and ended up blasting the corpses into the air, and they all fell down on him and buried him. After that, my dwarves tried to get the injured people water but it was winter, so no luck there. Then Bax came, a one-eyed dimetrodon with wings. It seemed pretty weak, because it was having a hard time fighting a troglodyte. I sent two dwarves to kill him, but they both got wrecked, so I decided to just wall off the entrance to the caves. I got a message a bit later, "Urist Mcdwarf cancels Construct Building, Interrupted by Forgotten Beast. Bax then began to massacre my surviving capable dwarves, and I sent a strong dwarf with a hammer to stop him. This dwarf got his arm torn off right away and Bax then picked up his hammer and smashed him to bits with it. After all my standing dwarves were dead, the ones in the hospital died of dehydration. I sent in a group of armed and armored dwarves to take care of Bax, and they were slaughtered as well. As the last dwarf was fighting Bax, I checked Bax in the unit menu and found that he has several weapons strapped to his body. Well, I give up on trying to defeat Bax. I'm pretty sure he's adapting and gaining weaponry to take over the world.