Muḥammad b. al-Ḥasan al-'Askarī (born in 255/869), also known as Imam al-Mahdi , is the twelfth Imam in Twelver Shi'ism. He is the promised savior, who will rise one day and fill the earth with peace and justice. Imam al-Mahdi (a) has been in occultation from the early years of his life. Shi'as regard him as the Imam of the present age. Among his well-known titles are Imam al-Zaman (the Imam of the time) and Wali l-'Asr (the guardian of the age)