Complex sentence: Anorther theory is that the name comes Rüstringen in Frisia, a land ruled by the Danish viking Rorik of Dorestad, who was suggested to be the same as Rurik of Novgorod.
Split into simpler sentences: Anorther theory is that the name comes Rüstringen in Frisia. Rorik who also is believed to be the same as Rurik of Novgorod.