Prophet Muhammed (s.a.w.) has said: “Even if the entire duration of the world’s existence has already been exhausted and only one day is left before Doomsday, Allah will expand that day to such length of time as to accommodate the kingdom of a person from my Ahlul-Bayt who will be called by name. He will fill out the earth with and justice as it will have been full of injustice and tyranny (by then)”
From this Hadith, it is clear to every Muslim that the twelfth Imam will reappear when this world is full of sins and injustice.
There are many signs mentioned by the Masumeen (a.s) on the reappearance of the 12th Imam. It is reported in Biharul Anwar that after the last pilgrimage the Prophet made, the Holy Prophet (s.a.w.) stood near the Kaaba, and called his people to listen to him. The Prophet said: “listen to me carefully so that you transmit these words of mine to those who are absent today.” The Prophet began, “My people, a time will come when kings and rulers will be tyrannical.” The Prophet also said that the payment of Zakat will be stopped.
According to the Masumeen (a.s), Muslim countries will seek aid from non Muslim countries. This is already evident in a number of Muslim countries. Another prediction that is come true is that 60 impostors will claim to be Prophets. By their attractiveness, their persuasion, and their personality, they will misguide the people. 58 false Prophets have already emerged since the days of the Prophet (s.a.w.). We have only two more to witness.
It is reported in Qayamat-e-Sughra citing “Oqdatud-Durr” that Hazrat Amir (A.S) has said that the Mahdi will not appear until one-third of the world population will die by being killed and one-third will die as a result of epidemics.
Adapted from: “A Short History of the Lives of The Twelve Imams”