Al-Andalus

The commonly missed condition first: you must START as an IBERIAN-culture Muslim - Andalusian, Castilian, Basque, Portuguese, Catalan, Visigothic or Suebi. Arab and Berber don't qualify, and this hidden requirement is behind most "why didn't it unlock" threads. The wiki route: in 867, replace the Umayyad Sultan with a custom Andalusian Muwalladi ruler (this also fixes his dire starting opinion). Surrender the scripted wars, take the independent Iberian counties before Asturias can, then the island counties, and reclaim the rebel duchies once strong. With all of Iberia controlled, take the Avenge the Battle of Tours decision and it pops. Stacks with Iberian Hostilities if Fate of Iberia is running.