I have heard of the Mahdi before, he is the redeemer of Islamsism, the guided one.
Some other facts about the Mahdi...
1. Was used as the basis of Frank Herbert's Dune series.
2. The current president of Iran is a Twelver, which means that he believes that the Mahdi is coming back soon, and like Christian armageddonist extremists ("Left Behind") he believes that nuclear war could hasten his return. This is one of the reasons people are so worried about Iran having nukes. Luckily, it appears as though some of the mullahs and politicians are having second thoughts about him, so he may be reigned in. I would hope so!