Nemesis was the goddess daughter of
Nyx who was, and still popularly is, the personification of retribution for evil. Her name almost certainly comes from nemo, 'to distribute': originally Nemesis was a
goddess who dealt or distributed appropriate gifts to her worshippers. Later she became more abstract and her 'distribution' was of anger at anything unjust.
Today, in such phrases as 'overtaken by Nemesis', she appears less specifically as a dealer out of retribution, but more generally as some kind of fate or unpleasant consequence that will come upon one if a condition is not fulfilled or a deadline not met. Her
Roman equivalent is Fortuna.
