The city of Ephesus is historically significant for hosting two major church councils: the First Council of Ephesus in 431 AD and the so-called "Second Council of Ephesus" in 449 AD.
The First Council of Ephesus, convened by Emperor Theodosius II, is recognized as the third ecumenical council of the Christian Church. The primary issue addressed was Nestorianism, a doctrine that argued that Jesus Christ existed as two separate persons—one human and one divine. The council condemned this teaching, affirming that Christ is a single person who is both fully divine and fully human. A major result was the formal recognition of Mary as Theotokos, meaning ‘God-bearer’ or ‘Mother of God.’ Nestorius, the principal proponent of the disputed doctrine, was removed from his position and his views condemned (Wikipedia)[https://en.wikipedia.org/wiki/Council_of_Ephesus], (Britannica)[https://www.britannica.com/event/councils-of-Ephesus], (GotQuestions)[https://www.gotquestions.org/Council-of-Ephesus.html].
This council had about 200 bishops in attendance and further deepened the split between the Eastern and Western branches of Christianity, severely impacting the Church of the East (OrthodoxWiki)[https://orthodoxwiki.org/Third_Ecumenical_Council].
Often called the "Robber Synod" or “Latrocinium,” the Second Council of Ephesus was convened in 449 AD, again by Emperor Theodosius II, under the presidency of Dioscorus of Alexandria. It was called to resolve ongoing Christological controversies, specifically the dispute over the teachings of Eutyches, which emphasized the single divine nature of Christ (monophysitism). The council supported Eutyches, overturned the previous condemnation of him, and deposed his opponents, including Flavian of Constantinople (Wikipedia)[https://en.wikipedia.org/wiki/Second_Council_of_Ephesus], (Britannica)[https://www.britannica.com/event/Second-Council-of-Ephesus].
The proceedings were highly controversial and were later nullified by the Council of Chalcedon in 451 AD, which condemned the decisions of this council and restored those who had been deposed. The Second Council of Ephesus is not recognized as an ecumenical council by the wider Christian tradition due to its irregular process and violent conduct (OrthodoxWiki)[https://orthodoxwiki.org/Robber_Council_of_Ephesus].
Additionally, Ephesus hosted an earlier synod in 190 AD. This gathering of Christian leaders sought to resolve questions about the date of Easter, but this council is not counted among the ecumenical councils (Britannica)[https://www.britannica.com/event/councils-of-Ephesus].
In summary: Ephesus was the site of two of the most pivotal church councils in Christian history, with the first in 431 AD defining the doctrine of Christ's unified personhood and the controversial second council in 449 AD, whose decisions were later overturned.