Complex sentence: Justinian I, then the Byzantine Emperor, commissioned a throne to be made as a gift for Maximianus becoming the new archbishop and to encourage Maximian and strengthen his authority because he was not always popular.
Split into simpler sentences: Justinian I, the Byzantine Emperor, commissioned a throne to be made as a gift for Maximianus. The gift was given to Maximian for becoming the new archbishop and to encourage and strengthen his authority because he was not always popular.