Al-Musta'in

Abū al-ʿAbbās Aḥmad ibn Muḥammad ibn Muḥammad al-Mustaʿīn bi-ʾllāh (Arabic: أبو العباس أحمد بن محمد بن محمد; 836 – 17 October 866), better known by his regnal title al-Mustaʿīn, was the Abbasid caliph from 862 to 866 during the period known as the "Anarchy at Samarra". Following the death of caliph al-Muntasir, the Turkic military leaders bypassed the claims of al-Mu'tazz and his brothers and selected al-Mustaʿīn, a nephew of al-Mutawakkil, as caliph.

Al-Mustaʿīn's reign was marked by the growing influence of the Turkish military officers in the affairs of the caliphate. His accession provoked unrest in Samarra and opposition from supporters of al-Mu'tazz, while defeats in campaigns in Armenia and Asia Minor and internal rebellions contributed to instability within the caliphate. In 865, conflict between rival military factions led al-Mustaʿīn to flee from Samarra to Baghdad, where he became the center of a civil war with al-Mu'tazz and his supporters.

After a prolonged siege of Baghdad, al-Mustaʿīn agreed to abdicate in favor of al-Mu'tazz in January 866. Although he was promised safe conduct and retirement in the Hijaz, he remained under guard in Baghdad and was executed later that year on the orders of al-Mu'tazz.

Source: Wikipedia — https://en.wikipedia.org/wiki/Al-Musta'in