Agamemnon married Spartan princess Clytemnestra, returned to Mycenae, defeated Aegisthus, and became king.
Though taken by force by Agamemnon, Clytemnestra became a blameless wife, giving birth to sons and daughters, including Iphigenia and Orestes.
As Agamemnon sailed to war on Troy, after sacrificing their daughter Iphigenia at Aulis, to retrieve his brother's wife Helen, stolen by Prince Paris, Aegisthus became the lover of Clytemnestra.
When Agamemnon returned to Mycenae after the war, and despite having been warned by Achilles's ghost, he was caught in nets by Aegisthus and Clytemnestra, who slaughtered him. Cassandra, king of Troy Priam's daughter, that Agamemnon had taken for himself as the end of the war was killed as well in the process.