Complex sentence: Caliph Al-Ma'mun dies after a 20-year reign and is succeeded by his half-brother Al-Mu'tasim as ruler of the Abbasid Caliphate, making Samarra his capital.
Split into simpler sentences: Caliph Al-Ma'mun dies after a 20-year reign. He is succeeded by his half-brother Al-Mu'tasim as ruler of the Abbasid Caliphate, making Samarra his capital.