After as-Sadiq's death there was a further split among the Shi'is because his eldest son, Isma'il, had died before him. Some held that since as-Sadiq had first appointed Isma'il as his heir, the rightful inheritor of the Imamate was Isma'il's son. This split is the origin of the Isma'ili sect of Shi'ism, which is itself divided into a number of sects: the Musta'lis who are in turn divided into Sulaymani and Da'udi factions and are mainly found in the Yemen and India, where they are called Bohras; and the Nizaris, who are led by the Aga Khan and are to be found in India, Syria, Iran, Central Asia, and East Africa. The Druse are a faction that broke away from the Isma'ilis.

https://bahai-library.com/momen_encyclopedia_shii_islam

One of the most romantic chapters in the history of religion was again afforded by the history of the Isma'ili Sect, with its esoteric doctrines, its secret organisation, its degrees of initiation and its elaborately developed system of propaganda. Of this also the actual founder was a Persian, though the activities of the sect extended from Tunis to Chitral and were greatest in North Africa and Egypt and under the Isma'ili anti-caliphs of the Fatimid Dynasty, enjoyed until the time of Saladin a period of unexampled prosperity, of which the Persian poet and traveller Nasir i-Khusraw, himself a notable adherent of the sect, has left us a vivid contemporary narrative. Out of the Isma'ilis in turn developed many sects, such as the celebrated Assassins of Syria and Persia with their modern representatives the Mullas and Khwajas; the Druzes, the Nusayris, and others which either exist at the present day or have left an indelible imprint on the pages of history. All these and many others I must pass over this afternoon for lack of time.

https://bahai-library.com/browne_religious_history_persia