Which uncompleted opera was written by the composer of Croquefer, ou Le dernier des paladins?
Croquefer, ou Le dernier des paladins: Croquefer, ou Le dernier des paladins is a one-act opéra bouffe by Jacques Offenbach to a French libretto by Adolphe Jaime and Étienne Tréfeu, first performed in 1857. Jacques Offenbach:  He is remembered for his nearly 100 operettas of the 1850s–1870s and his uncompleted opera "The Tales of Hoffmann".
The Tales of Hoffmann