The Atreids were the descendants of Atreus, a king of Mycenae. The family suffered appalling misfortune due to the actions of the Gods. Atreus' two sons were Agamemnon and Menelaus. Menelaus married Helen, whose kidnap by Paris was the trigger for the Trojan War. As well as being implicated in the original kidnap, various Gods interfered in the course of the war.
When preparing to sail to his brother's aid in this war, Agamemnon's fleet was stuck in the harbour due to a lack of wind. An oracle told Agamemnon the waters were becalmed because he had angered the Gods and advised him to sacrifice his daughter to appease them. He did so, but this triggered a cycle of family violence in which Agamemnon was killed by his wife and her lover, who were in turn killed by Orestes and Electra, the children of Agamemnon.