Stories of Achilles
Their are two different myths of Achilles. In one of the myths Achilles is placed in a fire to burn off all his mortal parts off after unsuccessfully making him immortal by his mother Thetis but the ceromony was interupted by his father Peleus. So, his ankle was never made invincible. In the second myth Achilles was dangled over the Styx river. Every part of his body touched the river except his ankle she held him by. So, that is why Achilles ankle is the only vulnerable part of his body.
ACHILLES' STORY, THE ILLIAD & THE TROJAN WAR
Achilles story begins when Calchas prophesizes that in order for the Greeks to take Troy, they would need the aid of Achilles. Thetis, Achilles mother knew that if Achilles went into this battle, he would die. She then sent young Achilles to Scyros where he was hidden away and disguised as a girl. Eventually, Achilles was caught by the crafty Odysseus, who placed some armor in with the women's things and caught Achilles as he was the only one to be fascinated by the hardware. Achilles then agreed to venture off with Odysseus to Troy, leading an army of his father's people, the Myrmidons, his tutor Phoenix and his close friend Patroclus.
Once at Troy, Achilles quickly showed himself to be an unbeatable warrior. Along the way, Achilles captured many towns in the Trojan territory. In the town of Lyrnessos, he took the woman Briseis as his own personal prize. In the midst of the war, Agamemnon, leader of the Greeks, was forced by an oracle of Apollo to give up Chryseis, his own prize. He then proceeded to take Briseis from Achilles to make up for his loss. This of course began the central plot of the Illiad. After Briseis was taken, Achilles was enraged and refused to fight. From there on out, the war turned in the Trojan's favor. The Greeks pleaded and begged Achilles to return to battle, but he refused. His best friend Patroclus then asked to fight in his place, wearing his armor. Achilles agreed to this and the next day Patroclus was killed and the armor of Achilles was taken by Hector, who thought Patroclus was Achilles.
Achilles then got into a fit of utter rage. I mean he was REALLY ticked off at the death of his best friend. Thetis then got Achilles new armor made by Hephaestus, and he returned to the war and slayed Hector. Achilles the proceeded to drag the body behind his chariot in front of Troy and refused to allow Hector's body to be properly buried. When King Priam secretly entered into the Greek camp to plead for the body, Achilles gave in and allowed him to take the body. This is one of the more intriguing parts of the Illiad as Achilles showed complete respect for Priam who for all intents and purposes was the enemy.
From there, Achilles went on to defeat many foes and fight heroically until finally Paris, with help from Apollo, shot Achilles with a poison arrow in the one place where he was vulnerable...his heel. This of course is where we get the expression "Achilles Heel" basically meaning your weakness. No matter how great you may be, you'll always have a weakness, your Achilles Heel.