In the Trojan War tradition, Achilles was called the son of the sea goddess Thetis.
Achilles was born to Thetis, one of the Nereids, and Peleus, the mortal king of the Myrmidons. Their wedding was attended by gods and became the setting for the dispute that led to the Judgment of Paris. Thetis remained deeply protective of Achilles and repeatedly intervened in his life.
In the Iliad, she asks Zeus to help the Trojans temporarily so that the Greeks will recognize how much they need her son. After Patroclus dies, Thetis obtains new armor for Achilles from Hephaestus. She also warns Achilles that killing Hector will bring his own death closer.
Later stories add the famous bathing in the River Styx, which supposedly made Achilles invulnerable except at his heel. That episode is not narrated in Homer’s Iliad, so it should be distinguished from the earlier epic tradition.