Wikipedia
The Fathites (alternately Aftahiyya, Fathiyya) are a now-defunct branch of Shia Muslims who were supporters of Abdullah al-Aftah ibn Ja'far al-Sadiq, believing him to be Imam after his father Ja'far al-Sadiq, the sixth imam of Shiism, in 766 CE. Abdullah's inheritance of the imamate was contested, with varying stories stating that either that he died within 70 days of his father, or that he was not sufficiently competent.
One faction of Fathites believed that Abdullah al-Aftah had a son, Muhammad ibn Abdullah al-Aftah, who inherited the Imamate. Others, however, believe Abdullah died without issue, and many Fathites later rejoined the Shia mainstream, becoming followers of Musa al-Kadhim, Ja'far's other son who was recognized as the 7th Twelver Shia imam.