Everything was going fine, just repelled a siege of goblins and collected the loot and bodies...then chaos from inside the fort.
Inglish Mebzuthfenglel Etomdorenlanzil Atzul, Blood Beast Planter was his name.
So apparently one of my dwarves was cursed with being a Blood Beast and just eliminated a 120+ strong fort on his own (not including war animals and other creatures). The description of him says that "this type of creature is found near the underworld", although the controlled dwarf was a grower that lived near the surface. Not sure how that works out. I didn't see any type of message prior to the grower transforming and slaughtering everybody. The last thing I did was open the third cavern layer and seal it off.
The damage report on him is a full screen worth of scars, broken bones, dents, fractures, bruises and is missing two pieces: the left leg (all the way to the upper portion) and his left hand. There are still 6 dwarves left alive and a couple of animals, although they are about to be finished by him (he's quite determined to kill EVERYTHING). Even missing a leg doesn't seem to have slowed him down. He's an unstoppable creature of poisonous blood and a lust for combat.
He basically just transformed and started tearing dwarf by dwarf into pieces and consumed their blood. Any thing that he attacked instantly made a cloud of miasma (rotted the flesh immediately), so I couldn't even see what was happening. I still don't even know where or how it came from. The grower in question never left the first 3 layers in all the time he lived in the fort.
I had embarked in an untamed wilds area, nowhere near any evil places. I didn't get into hfs or even find the candy, so running into something this impossible to fight is :/. He just appeared in the middle of the dining room and had his own party. I am greatly confused by this turn of events. Guess I have to start a new fort. I just don't even know how he even.
Edit: Last 2 dwarves alive finally managed to kill him, and then...
A VILE FORCE OF DARKNESS HAS ARRIVED!
Have fun with the blood you guys!
....He can resurrect himself it seems. I was just viewing over his corpse and then he came back to life (not a zombie or anything, so no necromancer hiding). He wiped out the siege party and then died again. Since he was a dwarf, the game ended when he re-died (nobody else was left alive).
What a crazy fort.