18th century Court composer Antonio Saleri did Wolfgang Amadeus Mozart a favor when he turned down a commission to compose a new work to celebrate the coronation of Leopold II, Holy Roman Emperor, as King of Bohemia. Mozart happily took it up and is said to have dashed it off in 18 days with the help of his pupil Sussmayr.
Saleri may or may not have been Mozart's deadly rival. Peter Shaffer's play Amadeus, which speculates that Saleri tried in vain to destroy Mozart's reputation, is a highly fictionalized account of their relationship.
The libretto written more than half a century earlier by Pietro Metastasio (1698–1782) is based on the life of Roman Emperor Titus and highlights his magnanimity in pardoning those who'd plotted to assassinate him. (The title translates into English as The Clemency of Titus).
The opera was first performed publicly on September 6, 1791 at the Estates Theatre in Prague and remained popular for many years after Mozart's death.
The Clemency of Titus was the first Mozart opera to reach London, receiving its première there at His Majesty's Theatre on March 27, 1806.