By the time of the Parthian dynasty, another branch of Mazdaism had developed which was particularly popular in Media: Zurvanism. Its name was derived from that of its supreme god, Zervan, who represents unlimited time. Once again, a cosmic battle between Good and Evil is represented, but unlike Zoroastrianism, it is Ormazd (Ahura Mazda) himself who battles with Ahriman and not Spenta Mainyu. According to Zurvanism, the history of the world will last a span of 9,000 years (some texts say 12,000), divided into cycles of 3,000 years. The first two periods, those of the reigns of Ormazd and Ahriman, represent an alternation of the reigns of Good and Evil, a concept which is completely absent from Zoroastrianism. In the final period, Light and Darkness battle with each other, creating a mingling of Good and Evil, until the final victory of Light. In this battle, Mithra plays the role of arbiter and mediator.