Aristodemus of Sparta was a Spartan, mentioned in Herodotus' account of the Battle of Thermopylae. Little is known of him other than the fact that he was sent home to Sparta because of an eye infection; was shunned by his fellow Spartans upon his return; and later charged into battle blindly into battle at Platea, resulting in his death. He is the man which Dilios is inspired by.
In the film 300, Aristodemus is presented as the character named Dilios, played by David Wenham. In the film, in contrast to the real historic account, he presumably lost his eye in battle, rather than as a result of an infection. Furthermore, he was sent back to Sparta not because he was deemed unable to fight (due to losing his eye), but because King Leonidas wanted him to use his exceptional oratory skills to tell everyone the tale of how the brave three hundred made their final stand. He is later seen leading the Greek army into battle at Platea, where his fate is uncertain.