Answer
Husayn succeeded Hasan ibn Ali as the Shia Imam. After Hasan's death in 669 CE, the Shia community split into two main branches: the Twelvers and the Ismailis. The Twelvers believe that Husayn was the rightful successor to Hasan and the first of the Twelve Imams, while the Ismailis believe that Isma'il ibn Jafar, Hasan's eldest son, was the rightful successor.