My last and first fort was prospering, having killed 1 FB, a feathered wasp, and 1 werebull, with minimal casulaties (~15).
We were protected by one of the first strange moods to happen in my fort: an artifact hatch cover, with many precious gems and decorations. This was at the bottom of my central staircase, ending in the cavern layer, bolted shut and forbidden, while stairs extended to to the first caverns, surrounded by traps.
After a while, a fearsome towering bloated fire-breathing ape with external ribs and antennae showed up, possibly as a god called by the grey langur captive, who was being kept and taunted untrained for the sins of his fellows: stealing lots and causing one death through frienly fire punches and infected face.
I let the cavern creatures, of which there were many, attack the ape. Though my death count went from 50 to 150 in a metter of months, and the entire underground was turned to flame, ash, blood, corpses and miasma, the FB was injured, and impaired.
It finally beat the cavern creatures down, and was left alone in the depths. Then I realised, at the begginning of a strange mood, in a tanner's workshop in some out-of-the way part of the fortress, that mokeys can open doors. Storming past the meagre traps, the ape came up and killed 1/4 of our population. A militia was sent in, brave yet underequipped soldiers, great in training but low in iron kit, and died to flames. The central hallway filled with somke, and my dwarves collected in the panic room. Somehow, the FB got past the drawbridge, and everyone attacking him did nothing. At long last, once the death, slaughter, and uncomfortable leather-trouser whipping was over, the FB went down to the grave layer, and my dwarf finished the artifact: a boar leather hood decorated with black zircons.
The save still lies, Tunecem, on a different computer to the one I use, with a mauled FB lying below 80 corpses and clouds of miasma, while 1 struggling legendary leathercrafter takes the harvest in... if I go back to that fort, the ghosts will surely join me.