Aegisthus was the result of an incestuous relationship between Thyestes
and his daughter Pelopia. Pelopia married her uncle Atreus, who did not know
of their relation, and he took Aegisthus as his own son.
When Aegistus later found out who he really was, he killed Atreus and soon became the lover of Queen Clytemnestra while her husband king Agamemnon was away at Troy.
Agamemnon had left Clytemnestra with a singer, and as long as the singer was present, Clytemnestra resisted Aegisthus. He then took the singer to a deserted island, and Clytemnestra was seduced.
On the kings' return after the ten-year war, Aegisthus helped Clytemnestra to kill Agamemnon, and after this ruled Mycenae together with her for seven years.
He was eventually killed by Agamemnon's son Orestes. .