Unfortunately, and to Ephialtes' dismay, Leonidas can only offer him the duties of clearing corpses from the battlefield and fetching water for the men, as Ephialtes' deformities prohibit him from fighting like a Spartan. Ephialtes turns on the Spartans when offered various luxuries by King Xerxes of Persia. Ephialtes tells Xerxes of a flanking route that his forces can use to surround the Spartan warriors, thus dooming the 300 to outright slaughter. This betrayal is the pivotal moment in the film, and it's made significantly worse by the fact that it's at the hands of a man who chose Persian luxury over Spartan honour.
