Ephialtes of Trachis is one of the characters in the film 300.
He is played by Andrew Tiernan.
Ephialtes is a deformed hunchback in exile whose parents ran away from Sparta. Hoping to redeem his dad, he volunteers to Leonidas to join the army. But Ephialtes is too weak to raise up his shield high enough so Leonidas rejects him.
Mad at this, he goes to Xerxes and betrays his own country to get a reward.
Ephialtes tells the Persians of the Anopaea, a path which only locals know. In real life, he was a simple traitor who never got the reward and was killed.