From someone else : A FB spawned in caverns, he/she sent the whole army at it. They all died. Then a stray cat killed it.
Hmmm... anti-climactic end to a climactic situation. I had an issue like that. After a particularly horrific siege my dwarves had all but killed each other in a fit of crazy (those who hadn't already died by their wounds). It eventually came down to a crazed, berserking axedwarf (trained) who chopped up the rest of my military (he killed the local hero in his sleep) and proceeded to kill the remaining adults, children and animals. I soon realized that there were only 3 adult dwarfs left:
A doctor, a bedridden marksdwarf and a psychotic axe-murderer.
If the two stable dwarves were to die there would be no stopping the axeman; the children of the fortress would be doomed. I carefully snuck the doctor down to a bunker I'd built for the dwarves. The marksdwarf was there as well, resting his grievous wounds in the barracks. My doctor had never carried a weapon before in his life, and he would surely be doomed, but he had no choice and went to suit up. If he fell, the wounded marksdwarf (and the fortress) would fall with them. The inexperienced doctor picked out a spear and several pieces of leather and bone armor.
Together, the doctor and the marksdwarf awaited their destiny.
And waited.
And waited some more.
...as it turns out, the axedwarf had died and would never hurt the fortress and its denizens ever again. I thought at first that he simply died of dehydration. After a bit of research I discovered, to my complete shock (and possibly awe) that this was not the case:
The axemurderer had been killed by perhaps the greatest warrior I never realized we had.
He was defeated... by a cow.
Apparently the dwarf approached to chop the cow into tiny bits and the animal just wouldn't have it. When the axedwarf took a swing the cow proceeded to bash his brains in with its mighty hooves. At last, after nearly a year of forgotten beasts, goblin sieges and psychokillers, the fortress was safe.
I then celebrated by barbecuing the fortress Messiah, turning him into a nigh impenetrable piece of leather armor. And I'd do it again.
Come to think of it... death by cow isn't as anti-climactic as I thought.