I've just seen a strange siege by the goblins. I got the "vile force of darkness" message alright, and closed the outside gates, so the goblins weren't going anywhere. The dwarves then ignored the outside world (who needs sunlight anyway?) and waited for the siege to go away...
...it didn't. For a long, long time, a whole year cycle at least. When I finally paid attention, I saw that only one goblin was listed, a Wrestler, and it was sitting still on top of a tiny one-tile mound at the very edge of the map. Then a Dragon turned up, and I opened the doors so I could capture it, but the goblins did nothing.
So I eventually sent some champions over to give the goblin a good talking to, which they did - except that after they'd killed it, another one appeared. So they killed that. Goblins then kept appearing, one by one, in the same spot, at intervals of about one second, each one surviving just long enough to be whacked about the head with a mallet. That little one-tile mound is now surrounded by arms, legs, heads and fifteen goblin corpses. Once the fifteenth goblin had died (one squad's worth, I guess) the "siege" message finally lifted.
It didn't seem to be that they were ambushing, or I'd've expected them to show up all at once, and I've never seen a siege last that long before. The only thing I did that was in any way unusual is that at one point I'd manually designated a lot of narrow iron gear to be melted, including some of the stuff which it turned out those goblins were wearing. I wonder if that made them stick around? Anyone seen anything similar?
All whacky-goblin-behaviour stories welcome.