The history of the imamate in Ismaili branch of Shiism may be traced to the dispute over the succession to Imam Jaʿfar al-Sadiq, who died in 148/765. According to the majority of the available sources, Imam al-Sadiq had originally designated his second son Ismaʿil, the eponym of the Ismaʿiliyya, as his successor to the imamate by the rule of the nass. This designation forms the basis of the claims of the Ismailis. According to the Ismaili religious tradition, Ismaʿil succeeded his father in due course. However, as related in the majority of the sources, Ismaʿil died before his father, and this raised questions in the minds of some of al-Sadiq’s Imami Shiʿi followers, who did not understand how a divinely guided imam could be fallible regarding so crucial a matter as his nass. A group of these Shiʿis had evidently already left Imam al-Sadiq’s following during his lifetime.2 It is not absolutely certain whether Imam al-Sadiq designated another son after Ismaʿil, although the later Twelver Shiʿis claimed such a nass for Musa b. Jaʿfar, the younger half-brother of Ismaʿil. At any rate, Ismaʿil was not present in Medina or Kufa at the time of Imam al-Sadiq’s death, when three other sons (ʿAbd Allah, Muhammad and Musa) simultaneously claimed his succession. As noted previously, this confusing succession dispute split the Imami Shiʿis into several groups, two of which may be identified with the earliest Ismailis, studying which can be a great help in knowing about the history of imamate in Ismaili branch of Shiism. These Kufan-based splinter groups had actually come into being earlier, as pro-Ismaʿil or proto-Ismaili factions of the Imamiyya, but they seceded from the rest of the Imami Shiʿis only after al-Sadiq’s death in 148/765.
One group, denying the death of Ismaʿil during his father’s life- time, maintained that he was the true imam after al-Sadiq, and they also held that he remained alive and would eventually return as the Mahdi. These Imami Shiʿis further believed that Imam al-Sadiq had announced Ismaʿil’s death merely as a ruse to protect him from the persecution of the Abbasids who were angered by his political activities. The Imami heresiographers al-Nawbakhti and al-Qummi call the members of this group, who recognised Ismaʿil as their Imam- Mahdi, the ‘pure Ismailis’ (al-Ismaʿiliyya al-khalisa), while some later heresiographers such as al-Shahrastani designate this group as al-Ismaʿiliyya al-waqifa, or those Ismailis who stopped their line of imams with Ismaʿil.
There was a second group of pro-Ismaʿil Shiʿis who played a significant role in the history of imamate in Ismaili branch of Shiism. Affirming Ismaʿil’s death during the lifetime of his father, they now recognised his son Muhammad b. Ismaʿil as their imam. The Imami heresiographers call this group the Mubarakiyya, evidently named after Ismaʿil’s epithet al-Mubarak (‘the blessed one’).5 It was only after al-Sadiq’s death that the majority of Ismaʿil’s supporters rallied to the side of Muhammad b. Ismaʿil and recognised him as their new imam. Soon after the recognition of Musa al-Kazim’s imamate by the majority of al-Sadiq’s followers, however, Muhammad b. Ismaʿil left Medina for the East and went into hiding to avoid Abbasid persecution, initiating the dawr al-satr, or ‘period of concealment’, in early Ismaili history which was to last until the establishment of the Fatimid caliphate.
Tayyibis and Nizairs:
After the establishment of Fatimid caliphate, the Mubarakiyya went through a lot of changes and this branch was split into different new sects itself among which the Tayyibis and Nizairs were famous.
The foundation of the Fatimid caliphate in 297/909 in Ifriqiya, North Africa (modern-day Tunisia and eastern Algeria), indeed marked the crowning success of the early Ismailis.
After the 11th caliphe of Fatimids, The Tayyibi Ismailis rejected the claims of al-Hafiz and the later Fatimid caliphs to the imamate. Tayyibi Ismailism found its permanent strong- hold in Yaman, where it received the initial support and protection of the Sulayhid dynasty.
The other sect called Nizaris also broke away from the Fatimid regime and they were preoccupied with a revolutionary campaign and their survival came to being in an extremely hostile environment. This sect is what the modern Ismailism is originated from.
Taken from: A History of Shiʿi Islam
by: Farhad Daftary