Achilles (myth)

Achilles was a hero in Greek mythology who fought in the Trojan War, according to Homer's Iliad. His only vulnerable spot was his heel, leading to the idiom "Achilles heel".

In 2267, Apollo said that Captain Kirk reminded him of the Greek heroes of old, including Achilles. 

In the mirror universe version of the $𝜇$, in the scene in which King Priam of Troy begged Achilles to return his son Hector's body to him for burial, Achilles killed him in cold blood rather than returning the body and allowing the funeral to take place. 

On Timshel, Leonard McCoy asked Kemal Marouk what the Joy Machine's Achilles heel was. 

McCoy later stated that Spock One's need to conceal his mirror nature was his Achilles heel. 

In one timeline, Kathryn Janeway considered the Borg's predictability to be their Achilles heel. (SS ST (A & Ω))

In one timeline, Alexander Rozhenko refused to be the Achilles heel of the Klingon Empire, but his father, Worf, did not get the reference. (SS TNG (Staying the Course))