Answer
The Zanj Rebellion, a major uprising of enslaved Africans in southern Iraq, was suppressed with the aid of al-Mu'tadid. The rebellion, which lasted from 869 to 883, posed a significant threat to the Abbasid caliphate. Al-Mu'tadid, who became caliph in 892, played a crucial role in quelling the rebellion by sending reinforcements and providing military support to the Abbasid forces.