Thyestes felt he had been deprived of the Mycenean throne unfairly by his brother, Atreus. The two battled back and forth several times. In addition, Thyestes had an affair with Atreus' wife, Aerope. In revenge, Atreus killed Thyestes' sons and served them to him unknowingly. After eating his own sons' corpses, Thyestes asked an oracle how best to gain revenge. The advice was to father a son with his own daughter, Pelopia, and that son would kill Atreus.
When Aegisthus was born, his mother was ashamed of her incestuous act. She abandoned him and he was raised by shepherds and suckled by a goat. Atreus, not knowing the baby's origin, took Aegisthus in and raised him as his own. Upon reaching adulthood, Thyestes revealed Aegisthus' true parentage, that he was both father and uncle to Aegisthus, who then killed Atreus and seized the throne.
Aegisthus and Thyestes ruled over Mycenae jointly, exiling Atreus' sons, Agamemnon and Menelaus to Sparta, where King Tyndareus gave the pair their wives, his daughters, Clytemnestra and Helen. When Tyndareus died, he gave his throne to Menelaus, who then helped Agamemnon overthrow Aegisthus and Thyestes.
When Agamemnon left Mycenae for the Trojan War, Aegisthus seduced his wife, Clytemnestra (daughter: Erigone), and plotted to kill her husband upon his return. They succeeded, killing Agamemnon and his new concubine, Cassandra. Eight years later, Agamemnon's son, Orestes, and his daughter, Electra, returned to Mycenae and killed both their mother, Clytemnestra, and Aegisthus.