Who is Aegisthus from The Oresteia and what is their importance?

Aegisthus is Clytemnestra's lover. As the only surviving son of Thyestes, he feels that he, not Agamemnon, should be governing Mycenae. Clytemnestra, however, actually does the dirty work while Aegisthus merely claims the rewards. Because of his having played a role in the murder of Agamemnon, Aegisthus is later killed by Orestes.