First a story, then a plea for suggestions on how to avoid it next time.
Welcome to Hailoils, year 5. Here's the rundown:
Population: 250, including soldiers. 95 were children, though.
Military:
-20 danger-room trained, unstoppable, full steel armed and armoured axedwarves (the whole shebang; 3 masterwork mail shirts, a breastplate, etc. etc., legendary every skill)
-50 mostly untrained marksdwarves wearing their clothes, crossbows, quivers, and shields. I draft most migrants into this force, and only use them when I'm under a serious siege (fortress defense mod is on) and have to seal the fort from the outside
This fort was one of my best yet. It had water death traps to drown the trade depot (ha! elves), magma pumped up near the surface for forges to crank out the huge amount of steel arms and armour necessary for my troops, eight legendary miners. it was awesome.
It also had periodic 'profane gloom' clouds, which turned anything they hit into thralls. This wasn't a big problem because I built a really twisty/airlocked entrance and didn't go to the surface much. It basically just led to the occasional barn owl or dingo getting thralled and attacking. I had an array of cage and weapon traps to deal with these.
Here's what happened:
A cloud shows up, so I order everyone inside. I don't bother sealing the entrance, because I've found it to be unnecessary. However, despite the burrow command, one of my axe lords runs outside, maybe to grab some gear or something. He gets turned into a thrall instantly.
me: meh. He's replaceable, and he'll just get cage trapped as soon as he... okay, nevermind. The traps ignored him completely. He sprints clear over the cage traps and crossbow traps and serrated steel disc traps, they don't trigger at all. I know that the profane gloom thralls aren't trapavoid because I've been catching wildlife, so what happened there? Are thralled dorfs immune to traps?
I keep my elite troops training near the entrance of the fortress, so he has to go through them to get inside. At this point, I figure things are bad but not so bad - the military will cut him to pieces, but I might lose one or two.
What happened: in a few seconds, the ~10 dwarves there are unconscious or asleep(!?) (but not dead) and the thrall enters the fort proper. I activate the marksdwarves, thinking a hail of *silver bolts* will put him in place. I have pillboxes where they can shoot into the entrance hallway without being in danger.
He ignores the bolts entirely, and the militia empty their quivers. At 25 bolts per dwarf and 60 guys firing, he shrugged off 1500 bolts. He has a bruised lip and broken rib after all of this, and that's all.
The other 9 axe lords are spread around the fort doing stupid tasks like drinking, sleeping, and upgrading their equipment, and trickle at him one by one to be knocked out or crippled. It appears that the thrall couldn't get through their steel armour, but broke enough bones and severed enough nerves that these dwarves were out of the picture.
During this period, of course, he's slaughtering all the civilians he comes across.
In ten minutes, he killed the entire fortress, except for the comatose axe lords. It was fast enough that there wasn't a single casualty to madness or tantrum spirals.
...How did this happen? Those axe lords have killed FB's and GCS's and entire sieges of war elephants single-handedly! I understand that the thrall gets extra strength, but even then the weight of numbers was firmly on my side, right? And why didn't my traps get him? I thought thralls didn't keep the skills of their prior lives, right? so, this was all perpetrated by a completely untrained axedwarf. To be fair, I guess he is the first person I've fought in this game wearing *steel armour*, but my dwarves had it too and I have occasionally lost them in battle. I've held off the circus with militaries less powerful than the one I had.