Complex sentence: Cassius had converted at the hands of the Arab, Hassan ibn Yassar al-Hudhali, qadi in Zaragossa at the time of Abd ar-Rahman's arrival in the peninsula, as a means to preserve his lands and political power.
Split into simpler sentences: Cassius had converted at the hands of the Arab, "Hassan ibn Yassar al-Hudhali", qadi in Zaragossa at the time of Abd ar-Rahman's arrival in the peninsula. He converted to Islam as a means to preserve his lands and political power.