Complex sentence: They contributed four monarchs listed in the "Nominalia of the Bulgarian khans"; according to the "Nominalia", they "ruled on that side of Danube for 515 years with shaven heads".
Split into simpler sentences: They contributed four monarchs listed in the "Nominalia of the Bulgarian khans". According to the "Nominalia", they "ruled on that side of Danube for 515 years with shaven heads".