...That goblins thread REALLY got off track, didn't it?
But yeah, I'd like parasites. Especially FBs carrying them. Imagine: You are attacked by your average, six-legged, furry, winged frog with external ribs and a stinger. It stings a few of your dwarves and domestic animals, as well as some cavern beasts, but won't further attack those it stings. This makes it easy to kill with your militia. However, a month later:
Urist McDwarf cancels Task: Transforming.
The stung dwarves, one by one, begin to warp. They sprout two extra limbs, their beards grow to cover their bodies, stingers spring out of their trousers, and they begin to grow. Soldiers are suffocated in their confining metal breastplates, leaving warped corpses, or burst their chainmail, leather armor, and clothes, turning them to useless piles of metal and leather and cloth. The new plague of forgotten beast spawn propagates their kind like their parent did, and swiftly transforms almost the entire populace into more forgotten beast spawn. The few survivors, walled into a farm plot and trying desperately not to accidentally release the monsters into their haven, know that they are the last hope of their fortress.
They set the clowns against the spawn, and watch as their fortress becomes a battleground between two of the deadliest forces of the underground. The single dwarf remaining after the fight, a child, watches from outside as an incoming caravan's guards finish of the last of the battlers, though it costs them dearly. The child, ignorant of the fact that she must now survive on her own in a deadly world, looks at the bloodstained depot and realizes that she has enough food now to last a lifetime. While no migrants join the fortress for years due to the danger and the well-timed sieges, the one remaining dwarf enjoys her life as the only dwarf in a once-prosperous fortress. She has the mayor's old room and the massive, gold-plated dining room to herself. Eventually, shortly after she reaches adulthood, a diplomat arrives and makes the fortress a barony. They accept and raise the only survivor of the greatest battle the world has ever known to the seat.
Awesome.