What was Achilles’ vulnerability according to Greek mythology?

In Greek mythology, Achilles is famously known for being nearly invincible due to his mother, Thetis, dipping him into the River Styx as a baby, which granted him immense strength and protection. However, during this process, she held him by his heel, leaving that part of his body vulnerable. This single point of weakness ultimately led to his downfall when Paris, aided by Apollo, struck him in the heel with an arrow. Consequently, the term "Achilles' heel" has entered the lexicon as a metaphor for a small but critical weakness in an otherwise strong character or entity, highlighting the importance of the heel as Achilles' vulnerability.