The Khawarij (collective plural ; multiple plural: ; singular ; literally "those who went out") are members of a sect that appeared in the first century of Islam and has manifested throughout the centuries since. It deviated from mainstream Islam and was known for killing Muslims under allegations of takfir (excommunication). It broke into revolt against the authority of Caliph Ali ibn Abu Talib after he agreed to arbitration with his rival, Muawiyah I, to decide the succession to the Caliphate following the Battle of Siffin (657). A Khariji later assassinated Ali, and for hundreds of years, the Khawarij were a source of insurrection against the Caliphate. Descendants of the original Khawarij survive today in small numbers, in more moderate forms.