Nemesis (noun)

1. A formidable and usually victorious rival or opponent.
2. A person's undoing.

Origin:

Late 16th century: from greek nemesis "retribution, goddess of divine punishment", from nemesisthai "dispense justice", from nemesios "dispensing justice", from nemos "distributor".

Examples:

1. He was finally able to defeat his nemesis in the tournament.
2. His nemesis was his own arrogance and lack of humility.
3. The nemesis of the superhero was the villainous mastermind.
4. The company's nemesis was the new competitor that had taken a big share of the market.
5. The nemesis of the athlete was the injury that prevented him from competing.