Complex sentence: Arshak III is often known as the last serving Roman Client King of Armenia in which the part of Armenia that Arshak III governed was under Roman rule from the Peace of Acilisene.
Split into simpler sentences: Arshak III is often known as the last serving Roman client king of Armenia. During his reign, the part of Armenia that Arshak III governed was under Roman rule from the Peace of Acilisene.