Dioscorus of Alexandria
Though some commentators like Anatolius and John S. Romanides think that Dioscorus was not deposed at Chalcedon (451) because of the faith, but for his grave administrative errors at the Robber Council of Ephesus (449), which included restoring Eutyches the heretic and the attack on Flavian, and because he (Dioscorus) had excommunicated Pope Leo of Rome, and also because at Chalcedon he refused to appear in front of the Council although he was summoned to it three times.
The Oriental Orthodoxy churches are generally accused by other churches of accepting the Eutychian doctrine of Monophysitism— this is denied by these churches as they consider Eutyches a heretic as the other churches but to have redemed himself by retreving this herecy in the Ephsus second council  but figures large in the differences between those churches and most other populous Christian churches, as well as in the civil strife and friction of the era and afterwards within the Eastern Roman Empire.
Hence, in the mess typical of schisms, according to mainstream Christian sects, he was merely a Patriarch of Alexandria turned heretic, who in a pre-emptive power-play characteristic of meglomania attempted to excommunicate many other influential bishops in opposition to his belief in Monophysitism, including Pope Leo I.
He was subsequently excommunicated by the Roman Catholic Pope Leo I, most likely in very early 450 AD during the aftermath of the controversial Second Council of Ephesus, which he was charged by the Emperor to precide over with the concurrence of Pope Leo I.
It was supposed to be the fourth ecumenical council and can only be discribed as Template:Wdy in effect and bizarre in it's rubber stamping character wherein giants of the orthodox sects were slain in abstentia by excomunication and which findings were all subsequently negated and annulled by Pope Leo I as well as the succeeding ecumenical council in 451 AD, the Council of Chalcedon (Widely accepted as the fourth ecumenical council, by most mainstream Christian sects. In contrast, the eastern sects listed above accept the Second Council of Ephesus as canonical, and don't accept the council of Chalcedon, nor the Chalcedonian Creed.)
The other person involved in this controversy apart from Dioscorus I is Leo I with each side considering the other person a heretic. The main factors behind this are still present and it is subject to discussion between the churches.
In recent research it was suggested that both Leo and Dioscoros are Orthodox because they agree with St.Cyril of Alexandria, especially with his Twelve Chapters, even though both had been considered heretical by the other side .
In May 1973 After fifteen centuries, H.H. Pope Shenouda III of Alexandria visited H.H. Pope Paul VI of Rome and declared a common faith in the nature of Christ, the issue which caused the schism of the church in the Council of Chalcedon  . Although, this is disputed, due to the fact that the main leaders of the Non-Chalcedonian schism specifically condemned St. Cyril's agreements with St. John of Antioch. For example, Timothy Ailouros (Dioscorus disciple and sucessor, wrote: "Cyril... having excellently articulated the wise proclamation of Orthodoxy, showed himself to be fickle and is to be censured for teaching contrary doctrine: after previously proposing that we should speak of one nature of God the Word, he destroyed the dogma that he had formulated and is caught professing two Natures of Christ" 
A similar declaration was reached between the Oriental Orthodoxy churches and the Eastern Orthodoxy churches in the 1990s. In the summer of 2001, the Coptic Orthodox and Greek Orthodox Patriarchates of Alexandria agreed to mutually recognize baptisms performed in each other's churches .
Before being a Pope Dioscorus served as the dean of the Catechetical School of Alexandria, and was the personal secretary of Saint Cyril the Great, Patriarch of Alexandria, whom he accompanied to the Third Ecumenical Council held at Ephesus.
In his struggle against Nestorius, St. Cyril explained the union between the two natures of Christ (His Divinity and His Humanity) as "inward and real without any division, change, or confusion." He rejected the Antiochene theory of "indwelling", or "conjunction", or "close participation" as insufficient to reveal the real unification. He charged that their theory permitted the division of the two hypostasis of Christ just as Nestorius taught.
Thus the traditional Orthodox formula adopted by Cyril and Dioscorus was "ONE INCARNATE NATURE" which translated in Greek to "MIA-PHYSIS" and not "Mono Physis". They meant by "MIA": one; not "single one", but "unity one"; "out of two natures"; as Dioscorus stated. He insisted on "the one nature" of Christ to assert Christ's oneness, as a tool to defend the Church's faith against Nestorianism. Thus Christ is at once God and man.
On the other hand the Antiochene formula was "Two natures after the union" which is translated to "DYO PHYSIS". This formula explained Christ as two natures; Son of God, and Son of Man, and that God did not suffer nor did He die.
St. Cyril himself, accepted the Antiochian formula, in his agreements with St. John of Antioch:
- "With regard to the Evangelical and Apostolic expressions concerning the Lord, we know that men who are skilled in theology make some of them common to the one Person, while they divide others between the two Natures, ascribing those that are fitting to God to Divinity of Christ, and those that are lowly to His Humanity. On reading these sacred utterances of Yours, and finding that we ourselves think along the same lines—for there is one Lord, one Faith, one Baptism—, we glorified God the Saviour of all"
A struggle occurred between Eutyches and Theodoret. Eutyches was an archmandrite of a monastery in Constantinople. He defended the formula "one nature" against that of "two natures". He concluded that the Godhead absorbed the manhood of Christ. Theodoret accused Eutyches and Cyril, and published a long attack on them. The council of Constantinople was held in 448, and Eutyches was condemned and exiled.
Pope Leo of Rome wrote to Eutyches praising his zeal in opposing the Nestorian dualism. But Leo changed his mind; perhaps when he heard that the emperor wrote to Dioscorus calling him to a council to be held to discuss that matter. Leo, who was not part of the conflict between the Alexandrian and the Antiochian Christology, sent the famous Tome (letter) of Leo to Constantinople -- not to work for reconciliation of the parties, but to defame the Alexandrian theologians.
Then Emperor Theodosius II convened the Second Council of Ephesus (called the "Robber Synod") in 449 and asked Dioscorus to exercise supreme authority over it as president. Eutyches was rehabilitated because he offered to repent and also because Leo, Bishop of Rome wrote to Flavian saying that he should be kind to him, and to accept him if he repented.
Then on July 28, 450, Emperor Theodosius died and his sister Pulcheria and her consort Marcian were declared emperors. Pulcheria supported Rome against Alexandria. She gathered signatures for the "Tome" of Leo to be introduced as the basic paper for a new council to be held at Chalcedon. At the same time, she decided not to let Rome hold supreme authority in the church. She refused Leo's demand to hold the council in Italy, but insisted that it would be held in the East. Although the council of Chalcedon is believed to have condemned Eutyches, the man with whom it really dealt was Dioscorus, for Eutyches was already in North Syria, where he had been exiled before the council met.
During the council, Dioscorus explained why the Orthodox faith should adopt the formula "One incarnate nature of God the Word". On hearing "one nature", some bishops in the council shouted, "Eutyches says these things also." Here Dioscorus clarified the Alexandrian view, saying, "We do not speak of confusion, neither of division, nor of change." Dioscorus tried to make his position clear: that he did not accept "two natures after the union", but he had no objection to "From two natures after the union."
When the judges started the order of the acts of the Council, Paschasinus the Roman delegate said, "We have orders from Rome that Dioscorus should not have a place in this council. If this is violated he should be cast out." When the judges asked about what Dioscorus did, the Roman delegate replied, "He has dared to conduct a council without the authorization of the apostolic see in Rome, a thing which has never happened and which ought not to have happened."
It was the emperor's favor that the council had to draw out Alexandria and declare a new formula to bring the entire Church in the east under the leadership of Constantinople. They used Leo as a tool to accomplish their objective through his enmity to Alexandria, looking upon it as an obstacle in realizing his papal authority on the Church over the world.
The verdict of the commissioners was announced: Dioscorus of Alexandria, Juvenal of Jerusalem, Thalassius of Caesarea, Eusebius of Ancyra, Eutathius of Berytus, and Basil of Seleucia - these were the men who had been responsible for the decisions of the second council of Ephesus, and should as such all be deposed. Thus the Patriarch of Alexandria was exiled to Gangra Island. In fact, Dioscorus was not condemned by name at chalcedon because of his theological heresy, but specifically due to his canonical violations at the Robber Synod of Ephesus.
New formula of faith
Under strong pressure, the bishops of the council accepted a new formula of faith, so that Alexandria would not acquire theological precedence. Yet when the delegates attempted to impose the papal authority upon the universal church, silence turned into revolt. Leo announced, in his repeatedly angry letters, his resistance to the council because it regarded Rome and Constantinople as equal.
Exile of Dioscorus
After those incidents, a messenger from Constantinople arrived in Alexandria announcing the exile of the Patriarch Dioscorus, and the appointment of an Alexandrian priest named Proterius as an imperial [i.e. alien/foreign/non-Egyptian] patriarch over Alexandria, with the approval of the emperor. He threatened whoever dared to show disobedience. The Melchite patriarch who was appointed by the emperor became surrounded by soldiers willing to punish those who might resist the imperial command.
In the year 457 Patriarch Dioscorus died in exile, and when the Copts heard that, they met with the clergymen and elected Timothy, the disciple of Dioscorus, to be the new Patriarch. This became a regular practice of the Coptic Church, who have not been reconciled to the Eastern Orthodoxy churches Patriarchates to this day.
- Catholic Encyclopedia
- Story of the Coptic church by Iris Habib elmasry Volume I
- Syriac Orthodox Church
- ORTHODOX AND ORIENTAL ORTHODOX CONSULTATION
- Coptic. net Monophysitism Reconsidered
- Timothy Ailouros, "Epistles to Kalonymos," Patrologia Graeca, Vol LXXXVI, Col. 276; quoted in The Non Chalcedonian Heretics, p. 13. See also"The History of the Persistant Monophysite Rejection of St. Cyril of Alexandria's Teaching on the Two Natures of Christ"
- Orthodox wiki
- John Karmiris, Dogmatic and Creedal Statements of the Orthodox Church, Vol. 1 [Athens:1960]. p. 154], quoted in The Non-Chalcedonian Heretics, p 11
- Encyclopedia Coptica: The Christian Coptic Orthodox Church Of Egypt
- Lives of Saints :: Tout 7 — 1. The Departure of Dioscorus, 25th Pope of Alexandria.
- List of Coptic Orthodox Popes of Alexandria
- Coptic Orthodox Church
- Orthodox Patriarchs of Alexandria
- Coptic Popes