Complex sentence: John was a scion of the Armenian Kourkouas family--a hellenized form of their original surname, Gurgen--which had risen to prominence in Byzantine service in the 9th century and established itself as one of the great families of the Anatolian land-holding military aristocracy.
Split into simpler sentences: John was a scion of the Armenian Kourkouas family which had risen to prominence in Byzantine service in the 9th century. The house of Kourkouas, a hellenized form of their original surname, Gurgen, had established itself as one of the great families of the Anatolian land-holding military aristocracy.