The caliph, following Hamdan's surrender, turns his focus to Harun himself.
Nasr al-Qushuri writes a threatening latter to the Kharijite leader, in an attempt to persuade him to submit; Harun, however, writes back a defiant response, rejecting Nasr's demands.
When al-Mu'tadid is shown the letter, he appoints al-Hasan ibn 'Ali Kurah over Mosul and charges him with combatting the Kharijites.
After crossing the Zab, al-Hasan encounters Harun and the two armies begin a grueling engagement.
The battle finally ends in defeat for the Kharijites; Harun flees to the desert and some of his followers surrender to the caliph.
