Twelver (, Athna‘ashariyyah or Ithna‘ashariyyah; , ) Shia Islam or Imamiyyah is the largest branch of Shia Islam. The term Twelver refers to its adherents' belief in twelve divinely ordained leaders, known as the Twelve Imams, and their belief that the last Imam, Muhammad al-Mahdi, lives in occultation and will reappear as the promised Mahdi.