Nemesis

[nem-uh-sis] /ˈnɛm ə sɪs/

noun, plural nemeses
[nem-uh-seez] /ˈnɛm əˌsiz/ (Show IPA)
1.
something that a person cannot conquer, achieve, etc.:
The performance test proved to be my nemesis.
2.
an opponent or rival whom a person cannot best or overcome.
3.
(initial capital letter) Classical Mythology. the goddess of divine retribution.
4.
an agent or act of retribution or punishment.
/ˈnɛmɪsɪs/
noun (pl) -ses (-ˌsiːz)
1.
(Greek myth) the goddess of retribution and vengeance
2.
(sometimes not capital) any agency of retribution and vengeance

1570s, Nemesis, “Greek goddess of vengeance, personification of divine wrath,” from Greek nemesis “just indignation, righteous anger,” literally “distribution” (of what is due), related to nemein “distribute, allot, apportion one’s due,” from PIE root *nem- “to divide, distribute, allot, to take” (cf. Old English, Gothic niman “to take,” German nehmen; see nimble). With a lower-case -n-, in the sense of “retributive justice,” attested from 1590s. General sense of “anything by which it seems one must be defeated” is 20c.
Nemesis [(nem-uh-sis)]

In classical mythology, the Greek goddess of vengeance.

Note: By extension, a “nemesis” is an avenger. One’s nemesis is that which will bring on one’s destruction or downfall.