Complex sentence: The Emirate of Aleppo and its neighbours became vassals of the Byzantines in the east, where the greatest threat was the Egyptian Fatimid kingdom, until the rise of the Seljuk dynasty reversed all gains and pushed Abbassid territorial gains deep into Anatolia.
Split into simpler sentences: The Emirate of Aleppo and its neighbours became vassals of the Byzantines in the east, where the greatest threat was the Egyptian Fatimid kingdom. However, the rise of the Seljuk dynasty reversed all Byzantine gains and pushed Abbassid territorial gains deep into Anatolia.