Complex sentence: The abbey site, including all its estates, were taken over by the counts of Stolberg who had exercised guardianship over the abbey since 1429 when the counts of Wernigerode died out.
Split into simpler sentences: The abbey site, including all its estates, were taken over by the counts of Stolberg. They had exercised guardianship over the abbey since 1429 when the counts of Wernigerode died out.