Imam Zaman (A.S): The Last Luminary
Therefore, by the reason of inner necessity and determination, the future will see a day when human society will be replete with justice, when all will live in peace and tranquillity, and when human beings will be fully possessed of virtue and perfection. The establishment of such a condition will occur through human hands but with Divine Assistance. And the leader of such a society is called in the language of the traditions, al-Mahdi (the Mahdi; the guided one).
In the different religions that govern the world (either divine or not) such as Hinduism, Buddhism, Zoroastrianism, Judaism, Christianity and Islam there are references to a person who will come as the savior of mankind. These religions have given glad tidings of his coming, although there are naturally minor differences in detail that can be discerned when these teachings are compared carefully.
However one thing that all these teachings have in common is that "A human will come to establish peace and tranquillity all over the earth". Every religion has a different attachment to it, however. The least thing that a human being (regardless of his religion) is supposed to do, is to accept what ALL these teachings have in common. This goes to prove the necessity of belief for "his coming". Thus, the saviors of all these religions who are all expected to come at the end of the time, are manifested in one individual, since the foundation of such belief is established by all. The attachments of different religions can be reconciled on their own tendencies, and thus rejected. The fact remains that the teachings of the previous religions have been under a long alteration process, and only Islam has been guaranteed its survival. Thus we should accept the attachment that the traditions of the Prophet (PBUH) has offered us with regard to the one who is coming whose name is Imam al-Mahdi (and, of course, there will come Jesus as one of the Imam al-Mahdi's follower, based on the prophetic traditions).
There are numerous traditions cited in the Sunni and the Shi'i sources from the Holy Prophet concerning the appearance of the Mahdi, such as that he is of the progeny of the Prophet and that his appearance will enable human society to reach true perfection and the full realization of spiritual life. In addition, there are numerous other traditions concerning the fact that the Mahdi is the son of the Eleventh Imam, al-Hasan al-Askari, and that after being born and undergoing a long occultation (disappearance; living among people while he is not identified) the Mahdi will appear again, filling the world with justice as it has been corrupted by injustice and tyranny.
Refrences: shia sources
Supplication and Salutation
Dua for The Twelfth Imam (A.J)
Special specifications of Imam Mahdi (AS)
Imam-e-Zaman (A.S.) in the Mirror of Dua-e-Nudba (Part 1)
Imam Zaman (A.S): The Last Luminary-part 1