When Helen was abducted by the Trojan prince Paris, Agamemnon led the Greek expedition to Troy to take back his brother's wife. In order for the Greek fleet to set sail from Aulis, Agamemnon sacrificed his daughter Iphigenia to the goddess Artemis.
When Agamemnon returned from Troy, he wasn't alone. He brought with him another woman as concubine, the prophetess Cassandra, who was famous for not having her prophecies believed. This was at least a third strike for Agamemnon as far as Clytemnestra was concerned. His first strike had been killing Clytemnestra's first husband, the grandson of Tantalus, in order to marry her. His second strike was killing their daughter Iphigenia, and his third strike was flagrant disregard shown for Clytemnestra by parading another woman in her home. No matter that Clytemnestra had another man. Clytemnestra and her lover (Agamemnon's cousin), killed Agamemnon. Agamemnon's son Orestes took revenge by killing Clytemnestra, his mother. The Furies (or Erinyes) took vengeance on Orestes, but in the end, Orestes was vindicated because Athena judged that killing his mother was less heinous that killing his father.
Pronunciation: a-ga-mem'-non • (noun)
Helen of Troy - The Basics
Who's Who in Greek Legend
Helen of Troy
Dioscuri - Castor and Pollux