Achilles (in ancient Greek: Ἀχιλλεύς), nicknamed swift-footed or swift-footed, is a legendary hero of Greek mythology, protagonist of the Trojan War described by the Iliad. Achilles was a demigod, son of the mortal Peleus, king of the Myrmidons of Phthia (region in the southeast of Thessaly) and of the Nereid Thetis. The myth of Achilles is among the richest and oldest in Greek mythology.

In the incomplete poem Achilleides by Publius Papinius Statius of the 1st century it is said that Thetis immersed Achilles in the river Styx to make him invulnerable, but holding him by the heel.

In Book IV of The Argonautics, Thetis, to make her son immortal, anointed him with ambrosia during the day, while at night, secretly from Peleus, she burned the mortal parts in the fire.

source: Wikipedia