Wikipedia
Abu Mansur Muhammad al-Qahir bi'llah , usually known simply by his regnal title al-Qahir bi'llah (, "Victorious by the will of God"), was the 19th Abbasid Caliph in Baghdad from 932 to 934. He was born 286 AH (899 C.E.) and died 339 AH (950 C.E.).
On the death of the former Caliph, al-Muqtadir, the courtiers, who feared that his son might revenge his father's death upon them, chose instead the late Caliph's brother al-Qahir; but he was an even worse lord than al-Muqtadir. With an outward affectation of godliness, he went to every excess of cruelty and extortion. He even tortured the mother of al-Muqtadir and his sons and favorites, to squeeze from them the wealth built up throughout the late reign. Many fled from his grasp.
The Caliph had his nephew, who was to have followed him, walled up alive. Al-Qahir, thus relieved from immediate threat, broke out into such tyranny, evenly against friend and foe, as to make his rule unbearable. A fresh conspiracy was begun under the former vizier Ibn Muqla, and the Caliph, overcome at night by wine, was set upon in his palace.
Refusing to abdicate, his eyes were blinded, and he was cast into prison in 322 AH (934 CE). Eleven years later he was freed, and was sometimes seen in beggar's rags and wooden sandals.