Formerly a monk, Anastasios 23 gave up the monastic life and returned to a secular career; described as dark of skin and of mind: Theoph. Cont. II 14 (p. 58) (μέλανα τὴν χροιάν, μέλανα τὴν ψυχήν); he was adopted by the rebel Thomas the Slav (Thomas 7) as his son and made a partner in his enterprise; this occurred in Thrace, before Thomas 7 made his first attack on Constantinople (Thomas 7's former partner, Konstantios 1, had been killed in Asia); Anastasios 23 was given command of the attack on the walls of Constantinople, separately from the main assault at Blachernai which was under Thomas 7 himself: Theoph. Cont. II 14 (pp. 58, 59), cf. Zon. XV 23. 2 (a former monk who preferred a secular career, he was adopted by Thomas 7 - υἱοθετήσατο δ'ἕτερον, τοῦ πρώτου κτανθέντος, ὡς εἴρηται, Ἀναστάσιον μὲν καλούμενον, μοναχὸν δ'ὄντα, καὶ τὸ μὲν σχῆμα τὸ θεῖον ἀποδυσάμενον, στολὴν δὲ κοσμικὴν μεταμφιασάμενον). After the final defeat and rout of Thomas 7, Anastasios 23 fled to the fortress of Bizye; he is described as ὁ δέ γε τούτου νόθος τε καὶ παρέγγραπτος υἱὸς Ἀναστάσιος: Theoph. Cont. II 18 (p. 68), cf. Zon. XV 23. 29. When news of Thomas 7's capture and death reached Bizye, he was bound and surrendered to the emperor Michael II (Michael 10), who inflicted on him the same punishments (mutilation and humiliation) and death as on Thomas 7: Theoph. Cont. II 19 (pp. 70-71), Zon. XV 23. 29.

According to Genesius, he was a former monk who had returned to the world and was adopted as his son by Thomas the Slav (Thomas 7) shortly before the attack on Constantinople; he was allegedly ugly, a drunkard (his drinking is said to have given him the appearance of a native of India - ἰνδογενής) and a stupid character: Genesius II 5. After Thomas 7 was defeated and fled to Arkadiopolis Anastasios 23, the false-named son of the rebel, held the fort of Byze (Bizye) and continued to resist the emperor (Michael 10); but, after the capture and death of Thomas 7, in mid October, the people of Byze fettered Anastasios 23 and surrendered him to Michael 10, who had him humiliated and executed in the same way as Thomas: Genesius II 8. The death of Thomas 7 was in mid October 823. Anastasios 23 was perhaps given the title of Kaisar by Thomas 7, as his adopted son and presumed eventual successor as Augustus.

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