nemesis

Merriam-Webster's Word of the Day for May 30, 2023 is:

nemesis • \NEM-uh-siss\  • noun

A nemesis is a formidable foe—an opponent or enemy who is very difficult to defeat. As a proper noun, Nemesis refers to the Greek goddess of vengeance.

// She will be playing against her old nemesis for the championship.

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Examples:

"2020’s original Enola Holmes proved to be a surprisingly enjoyable twist on the world’s most famous detective [Sherlock Holmes], focusing instead on his overlooked sister, Enola. No surprise, then, that this follow-up is just as exciting a romp through Victorian London. Despite proving her skills in the first film, Enola struggles to establish her own detective credentials until a missing-person report leads her to a case that’s stumped even Sherlock, and sees her crossing paths with his arch nemesis, Moriarty." — Matt Kamen, WIRED, 10 Feb. 2023

Did you know?

Nemesis was the Greek goddess of vengeance, a deity who doled out rewards for noble acts and punishment for evil ones. The Greeks believed that Nemesis didn't always punish an offender immediately but might wait generations to avenge a crime. In English, nemesis originally referred to someone who brought a just retribution, but nowadays people are more likely to see simple animosity rather than justice in the actions of a nemesis (consider the motivations of Batman’s perennial foe the Joker, for example).