nemesis

Concise Oxford English Dictionary © 2008 Oxford University Press:
nemesis /ˈnɛmɪsɪs/

▶noun (pl. nemeses /-siːz/)

• 1 the inescapable agent of someone's downfall, especially when deserved.

  ■ a downfall caused by an inescapable agent.

• 2 chiefly N. Amer. a long-standing or persistent rival or enemy.

– origin C16: Gk, lit. ‘retribution’, personified as the goddess of divine punishment.