Complex sentence: Within a few months, in April 892, al-Mu'tadid had his cousin removed from the succession--and probably killed--so that when al-Mu'tamid died in October, possibly poisoned, he succeeded to the throne as caliph.
Split into simpler sentences: Within a few months, in April 892, al-Mu'tadid had his cousin removed from the succession, and probably killed. As a result, when al-Mu'tamid died in October--possibly poisoned--he succeeded to the throne as caliph.