Complex sentence: The territory of the Free State of Saxony became part of the Holy Roman Empire by the 10th century, when the dukes of Saxony were also kings (or emperors) of the Holy Roman Empire, comprising the Ottonian, or Saxon, Dynasty.
Split into simpler sentences: The territory of the Free State of Saxony became part of the Holy Roman Empire by the 10th century. In the 10th century, the dukes of Saxony were also kings (or emperors) of the Holy Roman Empire, comprising the Ottonian, or Saxon, Dynasty.