The Mahdi is not to be confused with the Messiah Isa son of Mary (Jesus). They are two different persons, who will both appear in the Last Days. al-Mahdi will precede the second coming of the Messiah Isa son of Mary. The Messiah Isa (Jesus) will be able to slay ad-Dajjal (the false Messiah); al-Mahdi will not be capable of this.
al-Mahdi is a title meaning "the guided one". He will come from Makkah (Mecca) and rule from Damascus.
Khalif Ali, the fourth Khalif in Islam, reported that the Prophet Muhammad said, "Even if only a day remains for Qiyamah [Day of Judgment] to come, yet Allah will surely send a man from my family who will fill this world with such justice and fairness, just as it initially was filled with oppression."
According to other reported sayings by the Prophet Muhammad some of the features of al-Mahdi are:
The title has been claimed by various individuals in history, including Muhammad Ahmad who founded a short-lived empire in Sudan in the late 19th century.
Twelver Shia Muslims believe that the Mahdi is alive but in hiding. While he is not visible to humans, he is active in everyday life, and can be asked for help and will only reappear at the end of the world. For more information see: Muhammad al-Mahdi