1911 Encyclopædia Britannica/Aegisthus
AEGISTHUS, in Greek legend, was the son of Thyestes by his own daughter Pelopia. Having been exposed by his mother to conceal her shame, he was found by shepherds and suckled by a goat—whence his name. His uncle Atreus, who had married Pelopia, took him to Mycenae, and brought him up as his own son. When he grew up Aegisthus slew Atreus, and ruled jointly with his father over Mycenae, until they were deposed by Agamemnon on his return from exile. After the departure of Agamemnon to the Trojan war, Aegisthus seduced his wife Clytaemnestra (more correctly Clytaemestra) and with her assistance slew him on his return. Eight years later his murder was avenged by his son Orestes.
Homer, Od. iii. 263, iv. 517; Hyginus, Fab. 87.