 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 House of Atreus Act I: The House of Atreus Act I, released in 1999, is the ninth studio album by the American heavy metal band Virgin Steele. The House of Atreus Act I:  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. 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. Oresteia:  The only extant example of an ancient Greek theatre trilogy, the "Oresteia" won first prize at the Dionysia festival in 458 BC.
the Dionysia festival