Nemesis

/ˈnɛm.ə.sɪs/ noun easy

A long-standing rival or an agent of someone's downfall, especially one seemingly impossible to overcome.

The aging detective's true nemesis wasn't a criminal at all, but the department's own budget cuts.

Memory Trick

Nemesis was the Greek goddess of retribution — your nemesis is fated to bring your downfall, like divine payback.