Imam Hussein has said
We have twelve Imams, The first is Imam Ali and the last of them is my ninth child (generation), Imam Mahdi (A.J
God revives the earth when it is full of blasphemy and irreligious by him (Imam Mahdi).
Other religions will be overcome by pure Islam even if polytheists disagree.
In Imam Mahdi's occultation time, lots of people will apostatize and some will remain constant to Islam but they’ll be annoyed. This group of people will be disturbed by the people who ask them "If you are right so when is Imam Mahdi's reappearance time?"
(Imam Mahdi's reappearance time just determine by God).
Someone who endures these difficulties in his Occultation is the same as the person Crusades in Prophet Muhammad (PBUH) way.