Small kingdom (or province, or whatever) is suffer depredations by a tribe of orcs from the nearby mountains. The orcs are pretty vicious, and have slaughtered a lot of farmers and even completely wiped out a small village. Those they don't kill they take as slaves.
Heroes get to have fun killing orcs for a while, going off on various missions and being richly rewarded by the nobles for their efforts.
But, over time what they discover is that the nobles are secretly in league with (insert bandits or drow or drug dealers or Nazis or something) and have been provoking the orcs, and committing their own atrocities against them. The nobles have learned there is an ancient tomb, potentially stuffed to the gills with treasure, right smack dab in the orcs' stronghold, so they are hoping to motivate the populace to go to war and wipe out the orcs.