mahdian

from The Century Dictionary.

• noun One who holds that the Mahdi whose coming was foretold by Mohammed has already appeared; specifically, one who holds that the Mahdi has already appeared in the person of Mohammed Abu el-Qasim, the twelfth Imam, who is supposed to be concealed in some secret place awaiting the hour of his manifestation. The Shiahs in general hold this view. Also Mahdist.