Theognostos 20 was a protospatharios; he was one of the envoys sent to Charlemagne by the emperor Michael I (Michael 7), following the death of the emperor Nikephoros I (Nikephoros 8); the other envoys were Michael 6 and Arsaphios 3; they accompanied envoys sent by Charlemagne shortly before the death of Nikephoros I (Aio 1, Haido 1 and Hugo 1); in the church at Aachen they met Charlemagne and received from him the text of the ratified treaty, and they acclaimed him as Imperator and Basileus; on their way home afterwards they received another copy of the same agreement from the bishop of Rome, Leo III (Leo 11), in the basilica of St Peter: Annales Regni Francorum s.a. 812.

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