something that a person cannot conquer, achieve, etc.

The performance test proved to be my nemesis.

an opponent or rival whom a person cannot best or overcome.

( initial capital letter ) Classical Mythology . the goddess of divine retribution.

an agent or act of retribution or punishment.

noun

adversary , affliction , bte noire , curse , infliction , opponent , plague , rival , ruination , scourge , torment , archenemy , foe , antagonist , avenger , bane , destruction , downfall , enemy