Nemesis:

Background
Nemesis was known to be the daughter of Night though some call her the daughter of Erebus or Oceanus. She had two sets of twins Helen of Troy and Clytemnestra, and the Dioscuri and Polydeukes. Nemesis was the goddess of indignation against, and retribution for, evil deeds and undeserved good fortune. This goddess was thought to be the creator of an equilibrium for the people... she measures out happiness and unhappiness. He who is blessed with too many or too frequent gifts of good fortune, is paid a visit from her with losses and sufferings. The intentions behind her visiting them were so that he may become humble, and feel that there are bounds beyond which human happiness cannot go on without getting punished for. Most believe that gods were envious of excessive human happiness, and Nemesis was the main goddess involved evening out the happiness inside all mankind.

Present Day 
In today's day, the term "Nemesis" is thought of to be ones worst enemy. Usually someone who is the exact opposite of oneself. The word in the dictionary connects Nemesis with the phrases....

• source of harm or ruin
• retribution justice in its execution or outcome
• one that inflicts vengeance
• something causing misery or death

One's Nemesis is one who will bring on one's destruction or downfall.

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