Apollodorus Library 3.15.4

[4]
  Chione had connexion with Poseidon, and having given birth to Eumolpus ¹ unknown to her father, in order not to be detected, she flung the child into the deep. But Poseidon picked him up and conveyed him to Ethiopia, and gave him to Benthesicyme( a daughter of his own by Amphitrite) to bring up. When he was full grown, Benthesicyme's husband gave him one of his two daughters. But he tried to force his wife's sister, and being banished on that account, he went with his son Ismarus to Tegyrius, king of Thrace, who gave his daughter in marriage to Eumolpus's son. But being afterwards detected in a plot against Tegyrius, he fled to the Eleusinians and made friends with them. Later, on the death of Ismarus, he was sent for by Tegyrius and went, composed his old feud with him, and succeeded to the kingdom. And war having broken out between the Athenians and the Eleusinians, he was called in by the Eleusinians and fought on their side with a large force of Thracians. ² When Erechtheus inquired of the oracle how the Athenians might be victorious, the god answered that they would win the war if he would slaughter one of his daughters; and when he slaughtered his youngest, the others also slaughtered themselves; for, as some said, they had taken an oath among themselves to perish together. ³ In the battle which took place after the slaughter, Erechtheus killed Eumolpus.