Complex sentence: In the 6th century, the Slavs began to settle the area, but hereafter the history of Gorski kotar is obscure until the 12th century when the noble family Frankopans began to rule much of Gorski kotar.
Split into simpler sentences: In the 6th century the Slavs began to settle the area. The history of Gorski kotar is obscure until the 12th century when the noble family Frankopans began to rule much of Gorski kotar.