Complex sentence: In 1808 he became the cellist in the string quartet which Count Andrey Razumovsky had commissioned Ignaz Schuppanzigh to set up; he remained there until the quartet was disbanded after the Count's palace burnt down on New Year's Eve 1815.
Split into simpler sentences: In 1808 he became the cellist in the Schuppanzigh Quartet, which Count Andrey Razumovsky had commissioned Ignaz Schuppanzigh to set up. He remained with Schuppanzigh's quartet until it was disbanded, after the Count's palace burnt down on New Year's Eve 1815.