Complex sentence: Thomas the Archdeacon named him "the great" in his work "Historia Salonitana" during the 13th century for his significance in unifing the Dalmatian coastal cities with the Croatian state and accomplishing a peak in Croatia's territorial extent.
Split into simpler sentences: Thomas the Archdeacon named him "the great" in his work "Historia Salonitana" during the 13th century. He is particularly significant for unifing the Dalmatian coastal cities with the Croatian state, thus accomplishing a peak in Croatia's territorial extent.