Complex sentence: Until the end of the boy's regency in 1172, the kingdom saw turmoil which almost brought the ruling family down, although the reign of the second William is remembered as two decades of almost continual peace and prosperity.
Split into simpler sentences: Until the end of the boy's regency in 1172, the kingdom saw turmoil which almost brought the ruling family down. The reign of William II is remembered as two decades of almost continual peace and prosperity.