Answer
Al-Afdal Shahanshah, the vizier of Egypt, played a crucial role in the accession of Al-Musta'li to the Fatimid caliphate. Shahanshah manipulated the succession process, ensuring that Al-Musta'li, a young and inexperienced prince, became caliph instead of his older and more capable brother, Nizar. Shahanshah's support was instrumental in Al-Musta'li's rise to power, solidifying his position as the legitimate successor to the Fatimid dynasty.