What is the modern expresion that refers to a weak spot and based on figure from Greek mythology?

The modern expression that refers to a weak spot and is based on a figure from Greek mythology is "Achilles' heel."

This phrase originates from the story of Achilles, a powerful warrior in Greek mythology. His mother dipped him in the River Styx to make him invincible, but she held him by the heel, which remained vulnerable. This vulnerability led to his demise when Paris, a Trojan prince, shot him with an arrow in his heel.

Therefore, "Achilles' heel" symbolizes a person's or thing's weakness or vulnerable point, despite their overall strength or apparent invulnerability.