The reappearance of Muhammad al-Mahdi is the Twelver eschatological belief in the return of their Hidden Imam in the end of time to establish peace and justice on earth.
For Twelvers, this would end a period of occultation that began shortly after the death of Hasan al-Askari in 260 AH (873–874 CE), the eleventh Imam.
[3] Contemporary to the tenth Imam, the Abbasid al-Mutawakkil heavily persecuted the Shia,[4][5] partly due to a renewed Zaydi opposition.
[10] Tabatabai suggests that these restrictions were placed on al-Askari because the caliphate had come to know about traditions among the Shia elite, predicting that the eleventh Imam would father the eschatological Mahdi.
[16][14] This infant was kept hidden from the public out of fear of Abbasid persecution,[17] who sought to eliminate an expected child of al-Askari, whom persistent rumors described as a savior.
[28] The letter, ascribed to al-Mahdi, added that the complete occultation would continue until God permitted him to manifest himself again in a time when the earth would be filled with tyranny.
[34][36] In Twelver thought, al-Mahdi is also expected to avenge the injustices suffered by Husayn, grandson of the prophet, whose innocent blood is believed to have plunged the Muslim community into a cycle of violence, corruption, and oppression.
[41] Soon, however, the Hejaz, Iraq, the east, Egypt, Syria, and then Constantinople would fall to the army of al-Mahdi before the complete extermination of the forces of evil, after which the rule of justice would be established on earth and the humanity would be revived.
[37] Indeed, the belief in Mahdi remains popular among all Muslims, possibly owing to numerous traditions to this effect in canonical Sunni and Shia sources.
The foremost general sign of the second coming of al-Mahdi is the prevalence of evil on earth in the form of tyranny, injustice, and religious and moral degradation.