Did a quick search to see if there was anything like this, but I didn't spy anything similar, off-hand, so here we go.
So's I had a vile force of Darkness arrive earlier. Admittedly, it wasn't all that impressive... except for the fact that it was comprised entirely of ranged goblins (14 bowmen) and one swordmaster; I like to think the sword-gobbo was there just to say "Fire!' and wave his pointy blade around. Since my fort is isolated behind a big wall with a single five-tile passageway being the only way in or out - Double row of weapon traps and a final line of cage traps (to start with) protecting the interior side - a ballista pointed out just behind the cage-trap line - and a deep pit with a raising drawbridge over it blocking the way in, I was pretty safe to lure them in slowly. Downed about 7 of them, had to do something else, so saved to exit. Now, the siege was still going when I saved... Came back a while later, booted up the game, and the siege tag was gone. 'Huh,' thought I, 'They must be buggering off.'
Soon as I unpaused the game, I got that delightful message of a vile force of darkness arriving - This time, a more appropriate siege force. There was, in estimate, 40 goblins - Half hammerers, half bowmen, with a hammer lord, pikelord, and a elite crossbow-goblin leading the charge. ALL of them were mounted on jabberers, olms, giant rats, giant bats, voracious cave dwellers (or crawlers, If forget which,) even a giant toad in there somewhere and probably other critters I forget. The stranger thing was that there was trolls with them - That's cool, a pack of five trol- Wait, what the heck.
Another pack of trolls... with jobs. D: A trader, a siege operator, a craftsman, an animal trainer, and I think something else. I could only think of two things: These guys were really in it for the long haul and were going to pitch camp and wait me out, or I had a lot of misunderstood migrants.
The strange thing, though, is the double-siege. This a new feature? A little bug? Was the initial group of bowmen just a fore-runner for the real evil army?