Please answer a question about the following article about Imamah (Shia doctrine):

Although all these different Shia tariqahs belong to the Shia group (as opposed to the Sunni group) in Islam, there are major doctrinal differences between the main Shia tariqahs. After that there is the complete doctrinal break between all the different Shia tariqahs whose last Imams have gone into Occultation and the Shia Nizari Ismailis who deny the very concept of Occultation. The Shia Nizari Ismailis by definition have to have a present and living Imam until the end of time.[citation needed] Thus if any living Nizari Ismaili Imam fails to leave behind a successor after him then the Nizari Ismailism’s cardinal principle would be broken and it’s very raison d'être would come to an end.

What has happened to some of the Shia tariqahs last Imams?
Occultation