After Achilles left the army the greeks kept losing their battles, then Patroclus Achilles best friend had a plan. Achilles wouldn't fight however he would let Patroclus wear his armor so the Trojans would be scared. The plan worked but Apollo was still mad at Agamemnon so he helped the Trojan prince Hector kill Patroclus.
Achilles was so angered by the death of his best friend so he chased Hector all the way to Troy and stabbed him in the throat!
Apollo who was still seeking revenge, told Paris (Hector's brother) that Achilles was coming to troy, so he shot an arrow at Achilles and Apollo made it hit his only vulnerable spot, the heel. Achilles then died on the spot.