Wow. I totally forgot this was happening. So yeah, King Leonidas from the movie 300 is in God of War: Ascension as a multiplayer character. You can play as the King of Spartans only if you pre-order the game from GameStop.
Pre-ordering Ascension from any other retailer will get you The Mythological Heroes Pack. This will let you play as Perseus, Orion, Odysseus, and Achilles.