Achilles' mother, Thetis, foresaw his death in Troy, so she disguised Achilles as a girl among the daughters of Lycomedes - King of Scyros, so he wouldn't have to battle in the war. He hid as a girl for years until Agamemnon found him. Odysseus devised a trick to procure the identity of Achilles by appearing as a merchant, selling many wares, and of these objects, a sword was included. The daughters of Lycomedes went to see the beautiful clothing and jewlery, but Achilles took up the sword; with this trick, Odysseus uncovered Achilles. Initially he refused to fight in the war, but after a convincing speech, he agreed. When they finally reached Troy he proved himself to be the greatest soldier in Greek history.
After 3 or 5 years, he withdrew from battle after he felt he was dishonored by Agamemnon. Agamemnon had taken a woman named Chryseis as his slave. Her father, a priest of Apollo, begged Agamemnon to return her to him. Agamemnon refused and Apollo sent a plague amongst the Greeks. The prophet Calchas knew the source of the troubles, but would not speak unless Achilles vowed to protect him. Achilles did so and Calchas declared Chryseis must be returned to her father. Agamemnon consented, but then commanded that Achilles' battle prize, Briseis, be brought to replace Chryseis. Angry at the dishonor (because he loved Briseis) and at the urging of his mother Thetis, Achilles refused to fight or lead his troops alongside the other Greek forces. At this same time, burning with rage over Agamemnon's theft, Achilles prayed to Thetis to convince Zeus to help the Trojans gain ground in the war, so that he may regain his honor.
As the battle turned against the Greeks, thanks to the influence of Zeus, it was declared that the Trojans were winning because Agamemnon had angered Achilles, and the king was urged to appease the warrior. Agamemnon agreed and sent Odysseus and two other chieftains, Ajax and Phoenix, to Achilles with the offer of the return of Briseis and other gifts. Achilles rejected all Agamemnon offered him, and simply urged the Greeks to sail home as he was planning to do.
The Trojans, led by Hector, subsequently pushed the Greek army back toward the beaches and assaulted the Greek ships. With the Greek forces on the verge of absolute destruction, Patroclus, Achilles' young cousin, led Achilles men into battle wearing Achilles' armor, though Achilles remained at his camp. Patroclus succeeded in pushing the Trojans back from the beaches, but was killed by Hector before he could lead a proper assault on the city of Troy. In a frenzy of grief, Achilles would have committed suicide, had his faithful companions not held him back from doing so. His mother Thetis then brought him new armor, forged by Hephaestus, and he forgave Agamemnon.
Enraged, Achilles brought down his wrath on the Trojan army, creating carnage among the Trojans. He filled up a river so full of bodies that it overflowed. But Achilles wanted one death - Hector's death. As the Trojans retreated to the safety of Troy, Hector bravely stood and fought Achilles. Achilles was victorious and proceeded to tie Hector's body to the back of his chariot and drag it around the city of Troy. Hector's father, King Priam, later had to go to Achilles' tent and beg for the body so he could bury it.
Achilles would not live for much longer, for an Oracle had predicted that if he slew Hector, he would die soon after. Paris, the abductor of Helen, came to the field and shot a poisoned arrow at his heel. The arrow was guided by Apollo, and Achilles fell and met his demise.
After his death, Ajax the Greater and Odysseus fought over the armor of Achilles. Odysseus was granted the armor and Ajax became mentally unstable, eventually committing suicide.
He seems to regret his choice when Odysseus meets him in the Underworld saying he would do everything and more for a few moments of life.