Complex sentence: Anonymus did not write of Simeon I of Bulgaria, Svatopluk I of Moravia, or other opponents of the conquerors known from contemporaneous sources; instead, he described the Magyars' fights with local rulers unreferenced in other sources, including Gelou, Menumorut, and Salan.
Split into simpler sentences: Anonymus did not write of Simeon I of Bulgaria, Svatopluk I of Moravia and other opponents of the conquerors known from contemporaneous sources. Instead, he described the Magyars' fights with local rulers unknown from other sources, including Gelou, Menumorut, and Salan.