just deserts; comeuppance; what one deserves
Used when someone gets a fitting negative outcome as a natural consequence of their own bad actions. Often carries a tone of 'serves them right'.
It's his just deserts that he got fired.
If you do that, you'll get what you deserve.
罰 means 'punishment' in a general sense, while 当然の報い specifically implies a deserved, natural consequence rather than an externally imposed penalty.
Phrase composed of 当然 (natural, deserved) + の (possessive particle) + 報い (reward, retribution). The meaning is transparent from its components.