Theoktistos or Theoctistus (; died November 20, 855) was a leading Byzantine official during the second quarter of the 9th century and the de facto head of the regency for the underage emperor Michael III from 842 until his dismissal and murder in 855. A eunuch courtier, he assisted in the ascent of Michael II to the throne in 820, and was rewarded with the titles of patrikios and later magistros. He held the high posts of chartoularios tou kanikleiou and logothetēs tou dromou under Michael and his son Theophilos. After Theophilos' death in 842, Theoktistos became member of the regency council, but soon managed to sideline the other members and establish himself as the virtual ruler of the Empire. Noted for his administrative and political competence, Theoktistos played a major role in ending the Byzantine Iconoclasm, and fostered the ongoing renaissance in education within the Empire. He also continued the persecution of the Paulician sect, but had mixed success in the wars against the Arabs. When Michael III came of age in 855, his uncle Bardas persuaded him to throw off the tutelage of Theoktistos and his mother, the Empress-dowager Theodora, and on 20 November 855, Theoktistos was assassinated by Bardas and his followers.
Early life
Nothing is known of Theoktistos' early life. He is called a eunuch in Theophanes Continuatus and al-Tabari and is generally accepted as such by modern scholars, although an accusation by his rival Bardas of wanting to marry Empress Theodora or one of her daughters appears incompatible with this. By 820 he held an unspecified position at the court of Emperor Leo V the Armenian (), possibly as a member of the imperial guard. Theoktistos played a major role in the plot to assassinate Leo, and was rewarded by the new emperor, Michael II the Amorian (), with the rank of patrikios, and the confidential court post of chartoularios tou kanikleiou ("secretary of the ink-pot"). Under Michael's son and successor, Theophilos (), he apparently continued to be a trusted advisor, as he rose to the rank of magistros, and was appointed logothetēs tou dromou, effectively the Empire's foreign minister. A further mark of imperial confidence was Theophilos appointing Theoktistos as a member of the regency council for his two-year-old son Michael III shortly before his death in January 842, alongside the empress-dowager Theodora, and the magistros Manuel the Armenian.
Regency
Following Theophilos' death, the regency council took over the conduct of affairs of state. Theodora's brothers Bardas and Petronas and her relative Sergios Niketiates also played an important role in the early days of the regency.
The regency moved quickly to end Byzantine Iconoclasm, which had dominated Byzantine religious and political life for over a century with deleterious effects. In early 843, an assembly of selected officials and clerics convened in the house of Theoktistos to form the Council of Constantinople. The council repudiated iconoclasm, re-affirmed the decisions of the 787 Second Council of Nicaea, and deposed the pro-iconoclast patriarch John the Grammarian. In his stead was elected Methodios I, who had been imprisoned by Theophilos for his iconophile beliefs. This event is commemorated as the "Triumph of Orthodoxy" by the Eastern Orthodox Church ever since. Theoktistos played a major role in these events. He is credited by almost all sources—Theophanes Continuatus, Genesios, John Skylitzes, and Zonaras—as a driving force behind the restoration of the icons, and particularly behind the deposition of John the Grammarian. He is commemorated as a saint by the Orthodox Church on 20 November.
A week after that, Theoktistos and Sergios Niketiates were sent on a campaign to recover Crete, which had been conquered in the 820s by Andalusian exiles. The expedition at first went well, as the Byzantine army landed and took control over most of the island, confining the Andalusians to their capital, Chandax. At this juncture, Theoktistos heard a rumour that in his absence, Theodora intended to raise her brother Bardas to the imperial throne. He hastily abandoned the army under Niketiates and returned to Constantinople, only to find the rumours false. Once in Constantinople, news arrived of an invasion of Asia Minor by Umar al-Aqta, emir of Malatya. Theoktistos was sent at the head of an army to confront him, but the resulting Battle of Mauropotamos ended in a Byzantine defeat. At the same time, the expeditionary corps left in Crete was defeated and almost annihilated by the Andalusians, who killed Niketiates.