A spy joins the mercenaries. After stealing some valuable secrets the mercenaries have to chase down the spy, and kill him.
. . . but the spy has friends. Along the hunt the mercenaries meet groups of ferocious horde that must be slain in order to find the spy. Each group has a letter that has the last known location of the spy. The spy must be stopped within a certain amount of time or there will be some sort of consequence.
So, sound like something possible to Roleplay? Only trouble I see is getting a lot of people online for this.