Achilles refuses to participate in the Trojan War because King Agamemnon insisted that he was the one who needed to own Achilles's slave and concubine Briseis, as he was forced to release his own slave, Chryseis. When Achilles finds out that his closest and most trustworthy friend, Patroclus, is dead, however, he becomes so enraged that he decides to return to battle and do whatever it takes to avenge Patroclus. He learns that Apollo, the Greek god of the sun and medicine, helped Hector kill Patroclus on the battlefield, which is why Achilles makes it his mission to kill Hector.
Thus, the pain, grief, and anger that Achilles feels after he loses Patroclus is what finally motivates Achilles to forget about his conflict with Agamemnon and return to battle, despite being told that he will certainly die if he fights. Achilles's love for Patroclus and his thirst for revenge, however, are much greater than his fear of dying; he kills Hector and drags his body around the walls of Troy and around Patroclus's grave. Achilles is ultimately killed by Paris, Hector's brother, as Paris shoots an arrow in his heel—Achilles's only vulnerable spot. By dying, Achilles fulfills the prophecy which the oracle made at his birth, that he will die as a young hero in Troy.