Achiles was a great warrior in fact he was named the greatest warrior in Homers Iliad. Not Homer Simpson by the way this Homer, was the great Greek epic poet who wrote the Iliad and the Oddysey.
Achiles was such a great warrior that it was believed he was unbeatable in battle but he ended up being killed by an arrow to the heel. The "Achiles Heel" became known as a man's principal weakness due to the fact that the great warrior Achiles was finally beaten in what some said was his only weakness.
David Beckham has now almost certainly been ruled out of this years World Cup because of a serious injury to his Achiles Tendon. The Achiles heel leads to yet another downfall.