nemesis

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A source of harm or ruin: Uncritical trust is my nemesis.
2. n. Retributive justice in its execution or outcome: To follow the proposed course of action is to invite nemesis.
3. n. An opponent that cannot be beaten or overcome.
4. n. One that inflicts retribution or vengeance.
5. n. Greek Mythology The goddess of retributive justice or vengeance.

Century Dictionary and Cyclopedia

1. n. In Greek myth, a goddess personifying allotment, or the divine distribution to every man of his precise share of fortune, good and bad. It was her especial function to see that the proper proportion of individual prosperity was preserved, and that any one who became too prosperous or was too much uplifted by his prosperity should be reduced or punished; she thus came to be regarded as the goddess of divine retribution. Sometimes Nemesis was represented as winged and with the wheel of fortune, or borne in a chariot drawn by griffins, and confounded with Adrasteia, the goddess of the inevitable.
2. n. Retributive justice.
3. n. [NL.] In zoology, a genus of crustaceans.
4. n. The 128th planetoid, discovered by Watson in 1872.

Wiktionary

1. n. An archenemy
2. n. The principle of retributive justice.
3. n. A punishment or defeat that is deserved and cannot be avoided.
4. n. The polar opposite of a character.
5. n. A righteous infliction of retribution manifested by an appropriate agent.

WordNet 3.0

1. n. (Greek mythology) the goddess of divine retribution and vengeance
2. n. something causing misery or death