A document of 27 May 912 in the Libro de los Testamentos that cites Flacinus is a twelfth-century forgery of the bishop Pelagius, not as it claims of a certain bishop Hermenegild.
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His predecessor was Gomelo II and he first appears in a document of the latter's episcopate, on 20 January 905, signing as both a presbyter and a primicerius ("Flacinus presbyter, Primicerius testis").