Contract Devil, eh? I hope you read the fine print. It's usually not good news.
In my campaign, I also recently introduced Contract Devils as part of the large city of Asmodeus worshippers my group are exploring. They openly run shops where you can go and obtain wishes. Of course, the contract is pre-written in black ink on a black piece of parchment.
Anyway, all my players know me too well to fall for that. They went and fought a pack of hellhounds chasing some escaped slaves, delved into the mine where the slaves were hiding, bypassed the puzzle room and disarmed the trap, then fought the elder earth elemental and freed the slaves.
As a reward, they were told to meet their NPC contact at dawn, a vigilante named the Silver Falcon. He took them on a chase sequence through the city to raid the temple of the records of the slaves, finding the details of one character's enslaved mother and another character's enslaved brother. They burned the archive to the ground on their escape, and the vigilante revealed himself to be one of the two brothers they're searching for.
So now the group's going to have a choice to make next session. They can either aid the uprising that's going to occur now the slave contracts have been destroyed, or they can flee. I'm gonna have to prep the mass combat rules for them if they plan to fight, but if they win it'll mean they deal the devil-worshipping nation a big blow, taking out one of their critical trade cities.
Of course, since it's a location of pretty significant importance, the Asmodeans will probably send whatever they can spare to defend it. I figure a series of five encounters should deal with the outcome in one session. First a mass combat with the freed slaves vs. the local city guard, then a tactical combat with the party vs. elite guards, then vs. devil guards, a second mass combat vs. reinforcements to defend the city, then a tactical combat vs. a boss devil and his minions that teleport into the city.
I'm just wondering if that's too much combat, but I can't really think of anything else that would suit besieging a city. Anyone have any experience or suggestions about alternative events that might occur where a group leads an army to assault a city other than a straight up combat encounter?