Aegisthus

Aegisthus (/ᵻˈdʒɪsθəs/; Ancient Greek: Αἴγισθος; also transliterated as Aigisthos) is a figure in Greek mythology. He was the son of Thyestes and his daughter, Pelopia. The product of an incestuous union motivated by his father's rivalry with the house of Atreus for the throne of Mycenae, Aegisthus murdered Atreus to restore his father to power. Later, he lost the throne to Atreus's son Agamemnon.

While Agamemnon was at the Trojan war, Aegisthus became the lover of the king's estranged wife Clytemnestra. The couple killed Agamemnon on his return. He became king of Mycenae for seven years before he was killed in his turn by Agamemnon's son Orestes.

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.

Thyestes raped Pelopia after she performed a sacrifice, hiding his identity from her. When Aegisthus was born, his mother abandoned him, ashamed of his origin, and he was raised by shepherds and suckled by a goat, hence his name Aegisthus (from αἴξ, male goat). Atreus, not knowing the baby's origin, took Aegisthus in and raised him as his own son.

...
Wikipedia