Complex sentence: He was the disciple of the Eighth Shia Imam Reza and is said to have been given the authority to take bayah on behalf of the Imam and even to choose his successor, which he did in the time of Imamah.
Split into simpler sentences: He was the disciple of the Eighth Shia Imam Reza. And it is said that he was given the authority to take bayah on behalf of the Imam, and even he was authorized to choose his successor, which he did in the time of Imamah.