Complex sentence: In the north, where Fakhr al-Dawla's sons ruled, the Buyid frontier also fell back, as the Ziyarids of Gorgan and Tabaristan permanently wrested themselves from Buyid control.
Split into simpler sentences: In the north, where Fakhr al-Daula's sons ruled, the Buyid frontier also fell back. The Ziyarids of Gorgan and Tabaristan permanently wrested themselves from Buyid control.