I ran this basic idea by my players the other night and got pretty much universal joy from them, so it looks like I'm gonna run with this idea.
I'm liking some of the ideas in this thread- I dig the idea of the pcs eventually encountering some surviving nobles, for instance, and maybe cleaning out and reclaiming an area or something.
I think the basic structure of the enemy will be a tremendously large alliance held together by the mutual hate of the human (? dwarven, eladrin, who knows) empire. But as they crush the empire, the humanoids start to fall to squabbling amongst themselves, and soon you have a pretty ugly sort of free-for-all going on.
I think the first game session, maybe the first two, will take place in a city in the path of the enemy army. It's coming, and everyone knows it- and tons of people are starting to flee (but where??)- but there's a lot of pro-war, "our empire is invincible" propaganda everywhere, and no one admits that the cause is lost, even though everyone knows it. The pcs' first couple of adventures could include things like:
-scouting out the advancing army before it reaches the city.
-a chase after a fleeing spy with a map to the secret escape route from the city. If the enemy gets that map, there's no hope of evacuation!
-a harrowing fight through said secret escape route, maybe while it floods or something equally dramatic.
-holding off a group of bad guys long enough for the young heir to the throne to be spirited out of the city.
-attempting to reach mounts or crucial supplies while the enemy sacks the city, and then to escape with them.
-attempting to reach and burn crucial information before the enemy reaches it. Alternatively, attempting to reach and destroy a crucial bridge, road, access point, or other strategic point before the enemy seizes it.
And that's before they even get to be on the run out of the city! I see a lot of potential adventure in this setup...
