This is the Council of Ephesus which started in Selcuk, Turkey. Started by Roman Emperor Theodosius II during 431 AD. Approximately 200-250 bishops attended each meeting.
This council was called because it was primarily concerned with the Doctrine of Nestoriamism. Every meeting they discussed things along the topic of Nestorianism, Theotokos, and Pelagianism. (7 sessions were held)
The final decision was rejecting Nestorianism as heretical and removed Nestorius from office.
They also confirmed the Nicene Creed and condemned Nestorianism. Along with that they excommunicated all the bishops who didn't take the decision. Also, based on the Nicene Creed they made the title Theotokos for Mary real.
This is what the city of Ephesus looks like today.