The Episode under Julian the Apostate
According to the synaxarion, before setting out on his military campaign against the Persians the emperor Julian sent a devil ahead to explore the route his army should take. Publius, by his foresight, perceived the emperor's intent. He stood in prayer with his hands raised, praying day and night, and by this unbroken intercession blocked the path of the demon.
The account relates that for ten days the evil spirit waited, unable to proceed, until the monk concluded his prayer; it then returned to the emperor and reported that it had been thwarted. Enraged, Julian vowed to take vengeance on the saint upon his return from the campaign, but he never fulfilled the threat, for he soon perished. The historical campaign to which the notice refers began in 363, and Julian died that same year during the Persian expedition, a circumstance the tradition reads as the swift undoing of his vow.
The notice closes with a final detail: after Julian's death, one of his military commanders distributed the emperor's effects and received monastic tonsure at the hand of Saint Publius, entering the ascetic life under the very monk the emperor had threatened.