In Greek mythology, Aegisthus was the lover of Clytemnestra, and son of Thyestes and Pelopia. Thyestes, having a long – time rivalry with his brother and king of Mycenae, Atreus, was advised by an oracle to have a son with his own daughter, Pelopia, who would then kill his brother. Thus, Aegisthus was born. Because his mother was ashamed due to her father raping her, she abandoned the child to shepherds, and was later taken under the protection of Atreus, without knowing the child’s true identity.
When Aegisthus grew up, Atreus sent him to kill Thyestes with a sword, which his mother had given to him before she left. This sword revealed the true identity of Aegisthus to Thyestes, and they both devised a plan against Atreus. Aegisthus returned to his uncle and killed him, taking the throne of Mycenae for him and his father. They exiled Atreus’ sons, Agamemnon and Menelaus, to Sparta, where they married Clytemnestra and Helen, daughters of King Tyndareus, respectively.
After the abduction of Helen by Paris and the Trojan War, Aegisthus seduced Clytemnestra and together, they killed Agamemnon upon his return from Troy. He continued to rule over Mycenae, but on the eighth year, Orestes, son of Agamemnon, avenged his father’s death by killing him.