In Islam, the Mahdi is the foretold redeemer who will come to the Earth before Judgment Day along Jesus, and cleanse the world of injustice. He is not mentioned in the Quran, only the Hadith. There is not uniform belief about when the Mahdi will return and what he will do when he arrives.
Beliefs about the Mahdi on which most Sunni and Shia agree are that he will be a descendant of Muhammad, he will have the name Muhammad, his return will coincide with the rising of the Antichrist, and there will be a solar eclipse in the month of Ramadan.
Shia Islam has a more developed doctrine about the Mahdi. Many Shia believe the Mahdi was born in A.D. 869 and was taken by Allah when he was five years old and will return in the end times as the prophesied redeemer. These Shias believe he will have black hair and dark eyes. They further believe he will come in an odd year and first appear in Mecca at the Kaaba. People will initial fear him, but eventually many will follow.