The Ummayad Strikes Back

Needs Legacy of Persia (owned AND enabled - a Paradox dev confirmed as much to a confused player), starting as any Umayyad - in practice the 867 Sultan of al-Andalus. The trick that makes it possible: the Sunni Caliphate title is destroyed if its holder loses all land. Conquer along the African coast to Krete for range, swear fealty to the Abbasids, Claim Throne the Empire of Arabia from inside, then release the Caliph to independence and strip him. Reach piety level 3 BEFORE the title is destroyed - otherwise someone else can recreate the Caliphate first - then take the restoration decision; prestige isn't required for it, so ignore truces and keep conquering. Shortcut variant: the game rule making the Abbasids Administrative hands you the Caliphate with the throne. (If this appears twice in the list, it's the same achievement under two spellings - one unlock covers it.)