Aegisthus | King of Mycenae

Aegisthus Greek Mythology
In Greek mythology, Egistus (Greek: Αἴγισθος, Aigisthos) was the son of an incestuous relationship between Thiestes and his own daughter, Pelopia. Atreus, Thiestes' brother, married Pelopia without knowing that Pelopia was his niece. When Egystus was born, Atreus accepted him as his own son, however Egystus discovered his true identity and, pressured by Tiestes, killed his adoptive father.

He was the lover of Queen Clytemnestra. With her, he killed her husband (and his cousin), Agamemnon, commander of the Greek armies at war. He intended to kill Orestes, Clytemnestra's son, by himself, but he was saved by his sister Electra, who knew the whole plot. He was murdered along with his mistress by the two, who avenged their father's death.

Egistus and Clytemnestra were the parents of Erigone, who had a (illegitimate) son with her half-brother Orestes, named Pentilo.