Who convened the First Council of Ephesus, 431 AD, and why? Who were the principles figures, and what was the conclusion?
The Council of Ephesus, AD 431, was convened by Emperor Theodosius II who was a friend of the Nestorius, Bishop of Constantinople, who was being accused of heresy by the Cyril Patriarch of Alexandria, who had orders written by Pope Celestine addressed to Nestorius declaring the he must desist in his heresy as embodied in the Constantinople Creed within ten days of receipt of the Pope's orders or be deposed (stripped of all Church authority).
The result of the Council, which was a devastating experience of intrigue and death, was that Emperor Theodosius decided to accept opposing rulings and deposed Cyril, Patriarch of Alexandria, along with others including Memnon, who were both kept in close confinement. However, a delegation from the Council, whom the Emperor eventually allowed to come to Court, was able to convince him that the "great council" decree was correct and Theodosius reversed his decision. Nestorius, Bishop of Constantinople was himself deposed, though allowed to return to his old monastery as a monk, and Cyril was reinstated and returned home to Alexandria amidst shouts of joy. Therefore, previously established orthodoxy was triumphant and Nestorius' teachings were condemned as heresy.
[For an in-depth history, see Council of Ephesus: The occasion and preparation for the council, New Advent.]