In OTL, the Bulgars of the Pontic-Caspian Steppe pushed through the Avars and eventually settled in Modern day Bulgaria after rampaging through the Byzantine Empire, establishing subsequent Empires as a result. However, instead of going West as per OTL, what if in an ATL they went East into the collapsing Sassanaid Empire? What would need to happen for this to be done and what would be the immediate effects of a Bulgar controlled Persia?