Atreus was the son of Pelops and Hippodamia, and the king of Mycenae in the Peloponnese. He married Aerope and had Agamemnon, Menelaus, and Anaxibia.

Atreus and his twin brother Thyestes were exiled by their father for murdering their half-brother Chrysippus in their desire for the throne of Olympia. They took refuge in Mycenae, where they ascended to the throne in the absence of King Eurystheus, who was fighting the Heracleidae. Eurystheus had meant for their stewardship to be temporary, but it became permanent after his death in battle.

Atreus vowed to sacrifice his best lamb to Artemis. Upon searching his flock, however, Atreus discovered a golden lamb which he gave to his wife, Aerope, to hide from the goddess. She gave it to Thyestes, her lover and Atreus' brother, who then convinced Atreus to agree that whoever had the lamb should be king. Thyestes produced the lamb and claimed the throne.

Atreus retook the throne using advice he received from Hermes. Thyestes agreed to give the kingdom back when the sun moved backwards in the sky, a feat that Zeus then accomplished. Atreus retook the throne and banished Thyestes.

Atreus then learned of Thyestes' and Aerope's adultery and plotted revenge. He killed Thyestes' sons and cooked them, save their hands and feet. He tricked Thyestes into eating the flesh of his own sons and then taunted him with their hands and feet. Thyestes was forced into exile for eating the flesh of a human. Thyestes responded by asking an oracle what to do, who advised him to have a son by his daughter, Pelopia, who would then kill Atreus. However, when Aegisthus was first born, he was abandoned by his mother, who was ashamed of the incestuous act. A shepherd found the infant Aegisthus and gave him to Atreus, who raised him as his own son. Only as he entered adulthood did Thyestes reveal the truth to Aegisthus, that he was both father and grandfather to the boy. Aegisthus then killed Atreus.

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