In Greek mythology, Eumolpus was the son of Poseidon and Chione (or Hermes and Aglaulus). Chione, pregnant with Eumolpus, was frightened of her father's reaction so she threw the baby into the ocean. Poseidon looked after him and brought him to shore in Ethiopia, where his sister, Benthesikyme, raised the child, who then married one of her daughters. Eumolpus however loved a different daughter and Benthesikyme sent him in exile to Thrace. There, he helped plan a plot to overthrow King Tegyrios (who later forgave him on his deathbed and bequeathed his kingdom to Eumolpus). Eumolpus then fled to Eleusis.
During a war between Athens and Eleusis, Eumolpus sided with Eleusis. His son, Himmarados, was killed by King Erechtheus. In some sources, Erechtheus also killed Eumolpus and that Poseidon asked Zeus to avenge his son's death. Zeus killed Erechtheus with a lightning bolt or Poseidon made the earth open up and swallow Erechtheus.
Apollodoros : II,5,12 and III,15,1, 4; Homeric Hymn to Demeter 147, 474.