Complex sentence: Once the empire was attacked and eventually destroyed by barbarians under the leadership of Tarnum, a young knight Rion Gryphonheart founded Erathia on its ruins and built a fortress in which the few remaining residents of fallen empire could hide.
Split into simpler sentences: Once the empire was attacked and eventually destroyed by barbarians under the leadership of Tarnum. A young knight, Rion Gryphonheart, founded Erathia on its ruins and built a fortress in which the few remaining residents of fallen empire could hide.