Losing is fun!
I was looking for an excuse to post what happened in my fort last night, but that tragedy puts mine to shame.
As for the imps themselves, I finally finished them off with my mighty force of wrestlers. One did die (a new recruit - custom title "fresh meat") but the wrestlers handled the imps well overall. Once a dwarf gets his hands on the little bastard's throat they can't put out anymore fireballs - or if a legendary gets his hands on the imp it can longer breathe, move it's limbs, or do anything but pray for the sweet release of death.
I suppose I will mention what happened though...
One day the dwarves of the mountainhomes came to my fort and made a heart wrenching plea. They spoke of all the poor orphan children and how their lives would be made more bearable if only they had toys to play with. My mayor was moved deeply... to get another barrel of whiskey. The liaison then indicated that they would pay the maximum possible amount for any toys produced when they returned the next year. Thus did the dwarves set to producing a mighty assortment of wonderous toys. (cheap junk)
One year later, the dwarven caravan arrived - ready to collect the toys that would bring 'joy' to orphans everywhere. The dry grass crinkled beneath their wagon wheels and they sang a merry dwarven travel song. And then the fireball struck. The first wagon caught flame immediately, and the fire spread through the grass beneath their feet like... wildfire. The world burned and the flames spread until there was naught but ash and smoking corpses. One lone mule survived, his rear legs burned to the bone. He was driven to madness by pain and carnage. (He may still be alive)
The dwarves were saddened, of course, by the terrible loss of profits. But also because Urist McLumberdwarf had been especially industrious. He had cut down every tree in the entire forest - but the haulers were to busy (and frankly too scared of the vile imps) to collect them. Much were the imps cursed for the lack of beds and barrels in later seasons.