On the gothic medieval archway (14th century) in front of the Church of Saint John the Evangelist in Ravenna, one can see a depiction of the Empress Galla Placidia lying prostrate before the feet of Saint John the Evangelist, because the consecration of the basilica had appeared to her the night before whilst she was praying with the holy Barbaziano. Saint John vanished, leaving Galla Placidia with a sandal as a relic.
The Life of Saint Barbaziano, written in Ravenna, possibly in the 10th Century, tells us that having built the Church of St John the Evangelist, Galla Placidia was seriously worried because there were no relics of the Apostle’s body to be found.
So the Empress sought the advice of the priest Barbaziano, her confessor, and together, with vigils and prayers, they asked God to show them where to find the Saint’s relics.
During one of these so-called nocturnal ‘incubations’, Barbaziano was the first to see a figure in radiant robes, with an angelic face, blessing the church with an incense burner in his hand.
It was Saint John the Evangelist himself, and Barbaziano pointed him out to Galla Placidia.
She threw herself down to embrace his feet, but at that moment the Apostle vanished, leaving a pontifical sandal in the hands of the Empress. (From Corrado Ricci: Guide to Ravenna)