Al-Qahir was the Abbasid Caliph in Baghdad from 932 to 934.
On the death of the previous Caliph, al-Muqtadir, the courtiers afraid that his son might revenge his father's death upon them, chose instead the late Caliph's brother al-Qahir; and he proved even worse ruler than al-Muqtadir. With an outward affectation of piety, 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 treasures amassed during the late reign. Many fled from his grasp.
The Caliph caused his nephew, who was to have succeeded, to be immured alive. Al-Qahir, thus relieved from immediate danger, broke out into such tyranny, equally against friend and foe, as to render his rule unbearable. A fresh conspiracy was set on foot, and the Caliph, overcome at night by wine, was attacked in his palace.
Refusing to abdicate, his eyes were blinded, and he was cast into prison in 322 AH (934 CE). Eleven years after he was liberated, he was sometimes seen in beggar's dress and wooden sandals;—sad contrast to his high-sounding title, al-Qahir bi’llahi, "Victorious by the grace of God."
References
