 Abū <unk> Muḥammad ibn <unk> al @-@ <unk> ( Arabic : أبو إسحاق محمد بن <unk> <unk> ; October 796 – 5 January 842 ) , better known by his regnal name al @-@ <unk> bi ’ <unk> ( <unk> بالله , " he who seeks refuge in God " ) , was the eighth Abbasid caliph , ruling from 833 to his death in 842 . A son of Harun al @-@ Rashid , he succeeded his half @-@ brother al @-@ Ma 'mun , under whom he had served as a military commander and governor . His reign was marked by the introduction of the Turkish slave @-@ soldiers ( ghilmān or <unk> ) and the establishment for them of a new capital at Samarra . This was a watershed in the Caliphate 's history , as the Turks would soon come to dominate the Abbasid government , eclipsing the Arab and Iranian elites that had played a major role in the early period of the Abbasid state . Domestically , al @-@ Mu 'tasim continued al @-@ Ma 'mun 's support of Mu 'tazilism and its inquisition ( miḥna ) , and centralised administration , reducing the power of provincial governors in favour of a small group of senior civil and military officials in Samarra . Al @-@ Mu 'tasim 's reign was also marked by continuous warfare , both against internal rebellions like the Khurramite revolt of Babak Khorramdin or the uprising of Mazyar of Tabaristan , but also against the Byzantine Empire , where the Caliph personally led the celebrated Sack of Amorium , which secured his reputation as a warrior @-@ caliph . 
