Kitāb al-Ghayba, authored by the renowned Islamic scholar al-Shaykh al-Ṭūsī, is an indispensable resource on the concept of occultation in Shīʿa Islam, with a specific focus on Imam al-Mahdi (a). This narrative text revolves around Mahdawiyya, offering an in-depth exploration of the 12th Imam's occultation. The work was written around thirteen years before al-Ṭūsī’s death, it incorporates powerful dialectical arguments and Ḥadīths vindicating the doctrine of the occultation of the son of al-Ḥasan al-ʿAskārī (as), in addition to responding to other proto-Shīʿī sects such as the Wāqifa and detailing the prophesied signs of his advent. The book is meticulously structured into eight chapters. The book refutes claims about Imam al-Mahdī’s death and advent times and sheds light on his characteristics, attributes, and conduct. Kitāb al-Ghayba's historical and theological importance is well-recognized due to its thorough examination of Imam al-Mahdi's occultation. al-Shaykh al-Ṭūsī’s access to first-hand accounts and his profound understanding of Ḥadīths significantly enhance the book's credibility. Its comprehensive treatment of Mahdism and occultation makes it a reference point for anyone seeking to write on these subjects. It also critically evaluates the claims of individuals asserting to be deputies of Imam al-Mahdi, enhancing its relevance in the discourse around the topic.