Achilles like Ulysses was a great Greek mortal hero. His mother, Thetis was an immortal, who had caught the fancy of Zeus. But Zeus put a lid on his passion, when he learnt that a son of Thetis was destined to be greater than his father. Zeus then hastily got her married to a mortal hero Peleus. Thetis, never got over the fact that she an immortal had been forced to marry a mortal. Her worst fear was that her children would be mere mortals. Each time she had a child with Peleus, she threw her new born into the fire, hoping that the fire would melt away all that was mortal in her child, leaving the immortal part behind. However each time, the child just died. When the seventh son was born, and she was about to throw the child into the fire, her husband Peleus stopped her. This child was Achilles.
Achilles grew into a strong, brave and handsome boy. His mother loved him dearly, but she still worried about his mortality. One day, she took him to the river Styx that runs through the underworld and has the power to bestow immortality on those who take a dip in its waters. She held her boy upside down at the heels and dipped him from head to foot in the water. All of Achilles body thus became invincible, except for the heels, which had not been exposed to the water, and remained his vulnerable spot.
The boy grew up to be a great warrior and was called on for help on the side of the Greeks in the Trojan War. He fought bravely, but towards the end of the war, a poisoned arrow from the bow of Paris, hit Achilles in his heel, his vulnerable spot and he died. For all his mother’s efforts, Achilles died like a mortal.
The phrase Achilles heel has thus come to mean a weak spot in an otherwise strong unit.