It is the title of an opera seria in two acts by Wolfgang Amadeus Mozart – the last theatrical work of the Salzburg genius – set to music on a libretto by Caterino Mazzolà , in turn based on a 1734 melodrama by Pietro Metastasio . The first performance was held at the Estates Theatre in Prague on 6 September 1791 on the occasion of the celebrations for the coronation of Leopold II as King of Bohemia . The opera bears the number 621 in the Köchel Catalogue .