[ 'nɛmɪsɪs ]
■ noun ( plural nemeses -si:z )
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.