What is "al-musta'sim"

Al-Musta'sim Billah (full name: al-Musta'sim-Billah Abu-Ahmad Abdullah bin al-Mustansir-Billah; ; 1213 – February 20, 1258) was the last Abbasid Caliph in Baghdad; he ruled from 1242 until his death.

Al-Musta'sim , also known as al-Mu'tasim , served twice (first in 1377, then again in 1386–1389) as Abbasid caliph of Cairo under the tutelage of the Mamluk sultans.