When the last descendant of ali, the eleventh imam, disappeared in 830 c. e., many shia became twelvers, believing that the twelfth imam would

That he would return to be among the people in order to bring justice and peace.

Explanation:

Such a belief  plus the belief of the right of Ali as the rightful successor of prophet Muhammad as the leader of all Muslims, is one of the main differences between Sunni Islam and Shi´ia Islam, differences that have proved to be irreconcilable so far. The Twelfth Imam disappeared in the 9th century.

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