Complex sentence: Al-Mu'tadid, the "de facto" regent of the Abbasid Caliphate, removes his cousin Al-Mufawwad from succession, and becomes himself caliph after the death of Al-Mu'tamid, returning the capital from Samarra to Baghdad.
Split into simpler sentences: Al-Mu'tadid, the "de facto" regent of the Abbasid Caliphate, removes his cousin Al-Mufawwad from succession. He becomes himself caliph after the death of Al-Mu'tamid, returning the capital from Samarra to Baghdad.