 Complete the following phrase:  Ḥaydar ibn Kāwūs , better known by his hereditary title of al-Afshīn , was a senior general of Iranian descent at the court of the Abbasid caliphs and a vassal prince of Oshrusana. He played a leading role in the campaigns of Caliph al-Mutasim, and was responsible for the suppression of the rebellion of Babak Khorramdin and for his battlefield victory over the Byzantine emperor Theophilos during the 