Complex sentence: Isaac inaugurated his reign with a decisive victory over the Norman King of Sicily William II (the Battle of Demetritzes, 7 November 1185), who had invaded the Balkans with 80,000 men and 200 ships towards the end of Andronicus I's reign.
Split into simpler sentences: Isaac inaugurated his reign with a decisive victory over the Norman King of Sicily, William II, at the Battle of Demetritzes on 7 November 1185. William had invaded the Balkans with 80,000 men and 200 ships towards the end of Andronicus I's reign.