In 1258 the last 'Abbasid Caliph was crushed by the Mongol Hulegu. Several years before this fateful event, however, a rival Caliph was already proclaimed by the Sharif of Mecca in the streets of Tunis. This proclamation came in the form of a letter written by the Andalusi Sufi Ibn Sab'in "The Son of Seventy". The contents of this letter, buried in the Kitab al Ibar of Ibn Khaldun, left untranslated by nineteenth century orientalists, are filled with allusions not only to the Hafsid ruler, Al-Mustansir, but also to a unique vision of the nature of authority, Qur'anic allusions and Ibn Sab'in's philosophy of the unity of existence. This paper examines and analyses this so-called "Letter of Ibn Sab'in" to show the intimate link between new 13th century Sufi ideas and notions of power, authority and legitimacy in Arabia and North Africa. Inspired by hermetic, philosophical ideals, Ibn Sab'in wished to create in the Hafsid Caliphate a mirror of his vision of creation and the nature of God. Although Ibn Sab'in died in Mecca and would never return to Tunis, Hafsid claims to the Caliphate, made possible by his letter, would continue for centuries, despite the rise of rivals in Mamluk Egypt, Rasulid Yemen and elsewhere.