The House of Atreus Act I, released in 1999 is the first part of a metal opera inspired by the  only extant example of an ancient Greek theatre trilogy which won first prize at what festival?
Oresteia The Oresteia (Ancient Greek: Ὀρέστεια ) is a trilogy of Greek tragedies written by Aeschylus concerning the murder of Agamemnon by Clytaemnestra, the murder of Clytaemnestra by Orestes, the trial of Orestes, the end of the curse on the House of Atreus and pacification of the Erinyes. The trilogy—consisting of "Agamemnon" (Ἀγαμέμνων ), "The Libation Bearers" (Χοηφóρoι ), and "The Eumenides" (Εὐμενίδες )—also shows how the Greek gods interacted with the characters and influenced their decisions pertaining to events and disputes. The only extant example of an ancient Greek theatre trilogy, the "Oresteia" won first prize at the Dionysia festival in 458 BC. Many consider the "Oresteia" to be Aeschylus' finest work. The principal themes of the trilogy include the contrast between revenge and justice, as well as the transition from personal vendetta to organized litigation. "Orestia" originally included a satyr play, "Proteus" (Πρωτεύς ), following the tragic trilogy, but all except a single line of "Proteus" has been lost.
Medea (play) Medea (Ancient Greek: Μήδεια , "Mēdeia") is an ancient Greek tragedy written by Euripides, based upon the myth of Jason and Medea and first produced in 431 BC. The plot centers on the actions of Medea, a former princess of the "barbarian" kingdom of Colchis, and the wife of Jason; she finds her position in the Greek world threatened as Jason leaves her for a Greek princess of Corinth. Medea takes vengeance on Jason by killing Jason's new wife as well as her own children, after which she escapes to Athens to start a new life.