Aegisthus (or Aigisthos) was a figure in Greek mythology. He is known from two sources. The first is Homer's Odyssey, about the end of the 8th century BC. The second from Aeschylus's Oresteia, written in the 5th century BC.
As Agamemnon lay siege to Troy, his queen Clytemnestra took Aegisthus as a lover. The couple killed Agamemnon when he returned. Aegisthus ruled for seven more years before being killed by Agamemnon's son Orestes. This is part of the story of the Orestia.