Complex sentence: Emperor Theodosius II then convened the Second Council of Ephesus (called the "Robber Synod" by some opponents) in 449 and asked Pope Dioscorus to exercise supreme authority over it as president.
Split into simpler sentences: Emperor Theodosius II then convened the Second Council of Ephesus in 449, called the "Robber Synod" by some opponents. He asked Pope Dioscorus to exercise supreme authority over it as president.