Conflict with Eutyches
The defining struggle of Flavian's patriarchate concerned Eutyches, an influential archimandrite at Constantinople who taught that Christ's humanity was absorbed into his single divine nature. At the Home Synod of Constantinople on 8 November 448, over which Flavian presided with about forty bishops present, Eusebius of Dorylaeum laid charges against Eutyches, and the synod deposed him.
Eutyches appealed against the sentence and gained the support of Dioscorus of Alexandria. Emperor Theodosius II, by some accounts displeased with Flavian over his refusal to pay the customary gratuities to the powerful court eunuch Chrysaphius, convened a new council to reopen the matter.
The Robber Council and his death
The council that met at Ephesus on 8 August 449 was presided over by Dioscorus of Alexandria, who had received the presidency from the emperor, with about 130 bishops attending. It absolved Eutyches and reinstated him, while deposing and excommunicating Flavian together with Eusebius of Dorylaeum. According to testimony later given at the Council of Chalcedon, Flavian was forcibly seized, beaten, kicked, and whipped during the proceedings; tradition relates that monks led by Barsauma were among those who assaulted him.
Flavian was exiled to Hypaepa in Lydia and died on 11 August 449 from his injuries. Pope Leo I, whose doctrinal letter known as the Tome had been addressed to Flavian, protested the council and named it the Latrocinium, the "Robber Synod," nullifying its acts.
Vindication and veneration
After the death of Theodosius II in 450, the Emperor Marcian and the Augusta Pulcheria had Flavian's remains brought back to Constantinople in a triumphal procession. The Council of Chalcedon in 451 condemned Eutyches, confirmed the Tome of Leo, and recognized Flavian as a martyr.
Because Flavian reposed before the Council of Chalcedon, he belongs to the undivided Church and is honored across traditions; he is venerated in both the Eastern Orthodox and Catholic Churches. His memory is kept on 18 February.