Recently, a small group of warriors who claim to hail from Grindinar have attacked the Tai-Par training grounds. Very few people survived from the viscous attack- only two instructors and a handful of students escaped. Their mission is to return to the capitol city and warn the king of the impending invasion.
That could work, though I'd expand on it and make it more original to what you had in mind for the game instead of relying on something that's used a lot.