Terrifying biome, don't remember which (woodland anyway).
I just wanted to test a little what atrocities evil biomes had to offer and embarked casually as you would embark on your typical flux-volcano map. First thing I see after embarking is a group of Giant Wren (5 or 6 of them) wandering on the extreme north of the map and half-a-Wonbat corpse (it only had a spine and 2 legs left) on the other side.
I start to dig my hole quickly and set up some farms and stockpiles. Then, a dreadful mist cloud appears and slowly drift near my home. Fortunately for me, it avoided my fortress. Unfortunately for me, the Giant Wren group got hit. 2 of them became deadly thralls and started murdering the others meticulously. They then rushed in my fortress, scattering my dwarves around the map and chasing dogs. I quickly recovered and burrowed my dwarves inside while they were chasing the unlucky dog (which was eventually beheaded).
Then, the freakin' Giant Wren corpses (murdered by their friend, not so far from the fortress) come back to life and deal death in my lovely muddy hole. 3 more dogs and 5 dwarves die, the 2 others rushing outside, one to be ambushed by the Thralls, and the other, fleeing the Wren corpses, goes right to the half-Wonbat and get bitten to death.
Most epic slaughter I ever had.