Orestes

Orestes, the son of Agamemnon and Clytemnestra, and brother of Electra and Iphigenia, who killed his mother to avenge the murder by her of his father and went mad afterwards, but was acquitted by the Areopagus and became king of Argos and Lacedæmon; his friendship for Pylades, who married his sister Electra, has passed into a proverb; the tragic story is a favourite theme of the Greek tragedians.

Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)