Large numbers of Iranians refugees flee north from Iran after a messier conquest by the Caliphate. These Iranian tribes, bringing with them Zoroastrianism, convert some of the Turkic tribes of the North Caucasus. Meanwhile, the Caliphate, now the Ummayad Caliphate, has just sacked Constantinople and collapsed the Eastern Roman Empire. Within a few years however, a civil war caused by the death of the powerful and popular Caliph will cause a long period of turmoil and weakness in the Islamic Ummah, allowing the consolidation of Christianity in the southern Balkans, specifically the emergence of a strong, Byzantine-influenced Bulgarian state that will eventually retake the Bosporus and Constantinople.
In the Caucasus, the Zoroastrian *Khazarite is powerful, and has converted the majority of the local population away from Tengriism. However, a new Rus-dominated state, ruled from Kiev, is gaining in power. This Kievan Rus state will eventually conquer Khazaria. However, the pagan Rus, seeing the need to gain legitimacy with its new population, will convert to Zoroastrianism. This will lead to the dominance of Zoroastrianism among the Rus as their primary faith,