In Shīʿa Islam, even stronger beliefs surrounding al-Mahdī as the hidden Imām, who will emerge at the end of time, developed among the Twelvers (Ithna ʿAshariy(y)a). The twelfth Imām, ʿAli ibn Muḥammad Simmarī, was born in Samarraʾ in 869 (AH 255). On the death of his father in AH 260, he became Imām but was kept in seclusion (the first so-called occultation, ghaiba, ghaibat-i-sughra), being seen (if at all) on rare occasions only by senior figures. He answered questions through a succession of deputies (wakīl). Shortly before the death of the fourth wakīl in 939 (AH 329), it was announced that there would be no further Imām, that the major occultation would occur (ghaibat-i-kubra), and that the Imām would remain hidden until God gave him permission to manifest himself. Meanwhile, the hidden Imām gives guidance, hears prayers, and intercedes. al-Mahdī is also known as Imām-i-ʿAṣr (the Imām of the Period), al-Muntazar (the Awaited), and Ṣāḥib al-Zamān (the Lord of the Age). See also MUḤAMMAD AḤMAD (the Mahdi of the Sudan).
"al-Mahdī." The Concise Oxford Dictionary of World Religions. . Encyclopedia.com. (August 15, 2018). http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/al-mahdi
"al-Mahdī." The Concise Oxford Dictionary of World Religions. . Retrieved August 15, 2018 from Encyclopedia.com: http://www.encyclopedia.com/religion/dictionaries-thesauruses-pictures-and-press-releases/al-mahdi