Achilles was born to Peleus, king of Phthia in Thessaly, and Thetis, a sea-goddess of immense power. In Homer he is simply their son, mortal enough to die. Later poets, above all Statius in the Achilleid, told that Thetis, knowing the child was fated to die young, dipped him in the River Styx, the boundary between the living world and the underworld, holding him by the heel. Every part that touched the water became invulnerable; the heel she gripped remained exposed. That later story is the source of the heel, not the Iliad.

Thetis disguised the boy as a girl and hid him among the daughters of King Lycomedes on the island of Scyros, hoping to keep him from the war that called all Greek heroes to Troy. But the prophet Calchas had foretold that Troy could not fall without Achilles, and the wily Odysseus, the Greek hero famous for his cunning, discovered the disguise by laying weapons among the girls' gifts. Achilles reached for the sword and shield, revealing himself, and sailed for Troy knowing he would never return home.

For nine years, Achilles led the Myrmidons, his elite warriors from Phthia, in raids and battles across the Trojan plain. He was the best fighter among all the Greeks, the one they called "the best of the Achaeans." But in the tenth year, a quarrel broke out between Achilles and Agamemnon, the high king who commanded the Greek forces. Agamemnon had seized a slave-girl named Briseis from Achilles as compensation for having to return his own captive to her father, a priest of the god Apollo. Enraged at this dishonour, Achilles withdrew his men from the fighting and refused to return, even as the Trojans, led by the noble prince Hector, pushed the Greeks back to their beached ships and set them ablaze.

The crisis came when Hector, the greatest Trojan warrior and heir to the Trojan throne, broke through the Greek lines and threatened to burn the entire fleet. Achilles still refused to fight, but he allowed his dearest companion, Patroclus, to wear his armour and lead the Myrmidons into battle. Patroclus fought bravely and drove the Trojans back, but Hector met him in combat and killed him, stripping the divine armour from his body. When Achilles learned of Patroclus's death, his grief turned to a terrible fury. Thetis brought him new armour forged by the god Hephaestus, the divine smith, and Achilles returned to the battlefield a figure of terror. He cut through the Trojan ranks like a force of nature, filling the river Scamander with so many corpses that the river-god himself rose up against him. Then Achilles found Hector before the walls of Troy and killed him in single combat, dragging Hector's body behind his chariot in a savage display of vengeance that lasted twelve days.

Only when the aged King Priam, Hector's father, slipped into the Greek camp by night and begged for his son's body did Achilles relent. He returned Hector's corpse for proper burial, and for a brief moment the two enemies shared a meal and wept together, each mourning what the war had taken from them. But Achilles's own death was already approaching. The poems of the Epic Cycle and the later storytellers give the killing to Paris, a skilled archer but no warrior, with the god Apollo guiding the arrow. Roman poets fixed that arrow in the heel left dry by the Styx. He died on the Trojan plain, and his ashes were mixed with those of Patroclus in a golden urn. The Greeks mourned him as they had mourned no other, for with his death they knew the war would drag on without its greatest champion.

Achilles was buried on the Trojan coast, and his tomb became a site of pilgrimage for centuries. His armour, contested between Odysseus and Ajax son of Telamon after his death, went to Odysseus, driving Ajax to madness and suicide. The story of Achilles became the foundation of the Iliad, the epic poem by Homer that has defined heroic literature for nearly three thousand years. From the later heel-story the world took the phrase "Achilles' heel," a fatal weakness in an otherwise invincible body, and his story remains the template for the tragic hero who burns bright and dies young.