It's going to feature a physically enormous warlord (think Xerxes in 300) who has plans of not just ruling the kingdom, but of being acknowledged as a god. He's decimating his foes and nigh-unstoppable in battle.
|He looks like this guy.|
So what do the currently ruling powers (who very much would like to continue to be the current ruling powers) do? They hire the PCs to infiltrate the dude's place and learn what they can. They need to figure out how he knows their battle plans, if he has a sorcerer on his side, who he really is and where he came from. They want the PCs to poison his castle's well, and to break open his wine barrels, make as much of a problem as humanly possible without getting caught.
Of course, the PCs can get violent, too, if they want, but that'll be significantly more difficult than sneaking around and being subtle. It's all in how the PCs want it, I suppose.