Achilles, one of the most famous heroes of Greek mythology, was the son of Peleus and Thetis. Peleus was a mortal king of the Myrmidons, a group of warriors from Thessaly, while Thetis was a sea goddess, often associated with water and divinity.
The union of a mortal and an immortal led to Achilles being both a formidable warrior and possessing strength beyond that of ordinary men. According to myths, Thetis attempted to make Achilles invulnerable by dipping him in the River Styx; however, she held him by his heel, leaving that part vulnerable, which is where the term “Achilles’ heel” originates from.