and Artabasdos. Although they had not taken any action to prevent the overthrowal of Anastasius, they took issue with Theodosius' ascension, and Leo proclaimed himself Byzantine emperor in the summer of 716. He also sought the support of the Arabs, who viewed the Byzantine disunity as advantageous, and thought the confusion would weaken the Byzantine Empire and make it easier to take Constantinople. Theodosius negotiated a treaty with the Bulgarian khan Tervel (), likely to secure his support against an imminent Arab attack. The treaty fixed the border between the Byzantine Empire and the Bulgarian Empire at Thrace, ceding the Zagoria region to the Bulgarians, as well as stipulating the payment of tribute to the Bulgarians, the return of fugitives, and some trade agreements. Around this time, Sulayman had begun advancing into Byzantine territory, laying siege to Amorium, and a separate force entered Cappadocia. Negotiations with Leo led them to withdraw. Leo began to march his troops to Constantinople soon after declaring himself emperor, first capturing Nicomedia, where he found and captured, among other officials, Theodosius' son, and then marched to Chrysopolis. After his son was captured, Theodosius, taking the advice of Patriarch Germanus and the Byzantine Senate, agreed to abdicate and recognize Leo as emperor. Bury states that the elite of Constantinople, who might otherwise have sided with the inoffensive Theodosius, who would be unlikely to politically weaken them, sided with Leo, as Theodosius was not competent enough to deal with the Arab threat. He further states that the meeting of the Patriarch, senate, and chief officials, which chose Leo over Theodosius, was done with the knowledge and consent of Theodosius himself, who accepted the decision. Bury postulates that, without the threat of the Arabs, it is possible that Theodosius may have retained power, and a succession of nominal emperors might have followed him, controlled by court officials and the elites. Leo entered Constantinople and definitively seized power on 25 March 717, allowing Theodosius and his son, also named Theodosius, to retire to a monastery as monks. After his retirement to a monastery, Theodosius might have become the bishop of Ephesus, if he was the same person as Theodosius, son of Tiberius, in , and, if he is the same, died on 24 July 754 according to Sumner. Either he or his son is buried in the Church of St. Philip in Ephesus. Little is known of the reign