Nemesis
Etymology
From Nemesis, the Greek goddess of retribution who punished hubris and undeserved good fortune, restoring balance by humbling those who rose too high. That origin means a true nemesis is not merely a rival but an instrument of just downfall. Popular usage has flattened it to "longtime enemy," losing the sense of deserved punishment.
noun · /NEM-uh-sis/
An agent of downfall or retribution that one cannot overcome.
What it names
Use it for the force that delivers deserved comeuppance — richer than the loose sense of "archrival."
Example
Complacency, not the competition, proved to be the incumbent’s nemesis.
Related idea
A rival opposes you as an equal; a nemesis in the strict sense delivers the reckoning your own excess has earned — opposition versus retribution.