Can the demons walk off the map once they reach the surface? That could be amusing.
And now I come to think of it, shouldn't opening a portal to Hell be a siege trigger if you somehow survive that long? You'd think your neighbours would be a bit upset about that sort of thing.
Hell, make it be like, 6 siege triggers. Elves, humans, (maybe) goblins, and all of the beastmen races. Throw in a titan or two.
This idea doesn't really make a whole lot of sense to me. If anything, your neighbours should be sending
Reinforcements. Yeah, I'd be pretty pissed at the stupid dwarfs opening a portal to hell too. But you know, with all those demons swarming up out of that hole, I think fighting the *dwarfs* would be the last thing on my mind. And if the dwarfs actually plugged the hole, or successfully held all the demons at bay, then I wouldn't really have any incentive to invade. Hell, especially if they kept them at bay. You do not want to mess with the dwarfs who *won* the war against hell.
Maybe if it were possible to actually win against the demons, and neighboring civs had sustained damage over it, *then* they can consider military conflict\compensation demands. But these things should happen for reasons, and the only reason I can think for a civilization to siege you over letting the demons out, is if that civ is being *led* by a demon, one who has personal interest in seeing hell break loose. Even then that should be a test of his leadership and dominion, since 'demons are flowing out of the pit, let's help them!' should strain any demon lords cover.