 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?
the Dionysia festival
Title: The House of Atreus Act I
Passage: The House of Atreus Act I, released in 1999, is the ninth studio album by the American heavy metal band Virgin Steele.  It has the subtitle "A Barbaric-Romantic Opera".  This album is the first part of a metal opera inspired by the Oresteia, a trilogy of Greek tragedies written by Aeschylus which concerns the end of the curse on the House of Atreus.  The music was intended to be the soundtrack for theatrical shows, with actors impersonating the characters of the tragedy.  The metal opera was actually performed under the name "Klytaimnestra - The House Of Atreus" in European theatres from 1999 to 2001, with the production of the Memmingen Opera House company and Landestheater Production.
Title: Aegisthus
Passage: Aegisthus ( ; Ancient Greek: Αἴγισθος ; also transliterated as Aigisthos) is a figure in Greek mythology.  He was the son of Thyestes and his daughter, Pelopia.  The product of an incestuous union motivated by his father's rivalry with the house of Atreus for the throne of Mycenae, Aegisthus murdered Atreus to restore his father to power.  Later, he lost the throne to Atreus's son Agamemnon.
Title: Oresteia
Passage: 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.
Oresteia The House of Atreus Act I