From The Art and Popular Culture Encyclopedia
In Greek mythology, Achilles (also Akhilleus or Achilleus; Ancient Greek: was a Greek hero of the Trojan War, the central character and the greatest warrior of Homer's Iliad, which takes for its theme the Wrath of Achilles.
Later legends (beginning with a poem by Statius in the first century AD) state that Achilles was invulnerable in all of his body except for his heel. These legends state that Achilles was killed in battle by an arrow to the heel, and so an "Achilles' heel" has come to mean a person's principal weakness.
Achilles was the son of the nymph Thetis and Peleus, the king of the Myrmidons. Zeus and Poseidon had been rivals for the hand of Thetis until Prometheus, the fire-bringer, warned Zeus of a prophecy that Thetis would bear a son greater than his father. For this reason, the two gods withdrew their pursuit, and had her wed Peleus.
As with most mythology there is a tale which offers an alternative version of these events: in Argonautica (iv.760) Hera alludes to Thetis's chaste resistance to the advances of Zeus, that Thetis was so loyal to Hera's marriage bond that she coolly rejected him.
According to a fragment of an Achilleis— the Achilleid, written by Statius in the first century AD, and to no other sources, when Achilles was born Thetis tried to make him immortal by dipping him in the river Styx. However, he was left vulnerable at the part of the body she held him by, his heel. (See Achilles heel, Achilles' tendon.) It is not clear if this version of events was known earlier. In another version of this story, Thetis anointed the boy in ambrosia and put him on top of a fire to burn away the mortal parts of his body. She was interrupted by Peleus and abandoned both father and son in a rage.
However none of the sources before Statius makes any reference to this general invulnerability. To the contrary, in the Iliad Homer mentions Achilles being wounded: in Book 21 the Paeonian hero Asteropaeus, son of Pelagon, challenged Achilles by the river Scamander. He cast two spears at once, one grazed Achilles' elbow, "drawing a spurt of blood."
Also in the fragmentary poems of the Epic Cycle in which we can find description of the hero's death, Kúpria (unknown author), Aithiopis by Arctinus of Miletus, Ilias Mikrá by Lesche of Mytilene, Iliou pérsis by Arctinus of Miletus, there is no trace of any reference to his general invulnerability or his famous weakness (heel); in the later vase-paintings presenting Achilles' death, the arrow (or in many cases, arrows) hit his body.