Complex sentence: Napoleon was a great admirer, having first seen her in the title role of Gluck's "Iphigénie en Tauride" in 1802, and she was alleged to have been the emperor's mistress for a short time.
Split into simpler sentences: Napoleon was a great admirer, having first seen her in the title role of Gluck's "Iphigénie en Tauride" in 1802. She was alleged to have been Napoleon's mistress for a short time.