Complex sentence: The Quartetville website has been using "Quartet Project" miniatures to develop the "Quartet Project Challenge" concept which represents a series of free online masterclasses for players of all ages and abilities, world-wide.
Split into simpler sentences: The Quartetville website has been using "Quartet Project" miniatures to develop a "Quartet Project Challenge" concept. This concept represents a series of free online masterclasses for players of all ages and abilities, world-wide.