In 787 Ioannes 18 was basilikos ostiarios (a dignity held by eunuchs) and logothetes of the stratiotikon Logothesion (Ἰωάννου βασιλικοῦ ὀστιαρίου καὶ λογοθέτου τοῦ στρατιωτικοῦ λογοθεσίου); he and Petronas 4 were the two lay officials who attended the first five sessions of the Second Council of Nikaia (the Seventh Ecumenical Council): Mansi XII 999 (the first session, on 24 September), XII 1051 (the second, on 26 September; he spoke at this session, Mansi XII 1075; styled ὁ μεγαλοπρεπέστατος λογοθέτης), XII 1113 (the third, on 28 September; he spoke at this session, XII 1118; ὁ μεγαλοπρεπέστατος λογοθέτης), XIII 1 (the fourth, on 1 October), XIII 157 (the fifth, on 4 October). He is not mentioned at the sixth, seventh or eighth sessions. Possibly identical with either Ioannes 14 or Ioannes 522.

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