Complex sentence: At that time the Greek world was in turmoil owing to the actions of the Catalan Company, a group of mercenaries, veterans of the War of the Sicilian Vespers, originally hired by the Byzantine Empire against the Turks in Asia Minor.
Split into simpler sentences: At that time the Greek world was in turmoil owing to the actions of the Catalan Company. These were a group of mercenaries, veterans of the War of the Sicilian Vespers, originally hired by the Byzantine Empire against the Turks in Asia Minor.