Complex sentence: Opus 109 contains the popular pieces "Les Perles" (The Pearls) and "l'Orage" (The Storm); even more demanding are the 12 pieces of opus 105.
Split into simpler sentences: Opus 109 contains the popular pieces "Les Perles" (The Pearls) and "l'Orage" (The Storm). Even more demanding are the 12 pieces of opus 105.