Ubaydallah ibn Abdallah ibn Tahir was the last major Tahirid to hold high office in what dynasty?
Ubaydallah ibn Abdallah ibn Tahir Abu Ahmad Ubaydallah ibn Abdallah ibn Tahir (Arabic: أبو أحمد عبيد الله بن عبد الله بن طاهر‎ ‎ , ca. 838 – May 913) was a ninth century Tahirid official and military officer. He was the last major Tahirid to hold high office, having served as the governor of Baghdad at various points between 867 and 891.
Al-Mu'tasim Abū Isḥāq Muḥammad ibn Hārūn al-Rashīd (Arabic: أبو إسحاق محمد بن هارون الرشيد‎ ‎ ; October 796 – 5 January 842), better known by his regnal name al-Muʿtaṣim bi’llāh (المعتصم بالله , "he who seeks refuge in God"), was the eighth Abbasid caliph, ruling from 833 to his death in 842. A younger son of Harun al-Rashid, he rose to prominence through his formation of a private army composed predominantly of Turkish slave-soldiers ("ghilmān"). This proved useful to his half-brother, Caliph al-Ma'mun, who made use of al-Mu'tasim and his Turkish guard to counterbalance other powerful interest groups in the state, as well as employing them in campaigns against rebels and the Byzantine Empire. When al-Ma'mun died unexpectedly on campaign in August 833, al-Mu'tasim was thus well placed to succeed him, overriding the claims of his nephew, al-Abbas ibn al-Ma'mun.