The event described below by St. Adomnan (c. 690) in his Vita Sancti Columbae is said to have happened in 565 AD:
Also at another time, when the blessed man was for a number of days in the province of the Picts, he had to cross the river Nes [Ness]. When he reached its bank, he saw a poor fellow being buried by other inhabitants; and the buriers said that, while swimming not long before, he had been seized and most savagely bitten by a water beast. Some men, going to his rescue in a wooden boat, though too late, had put out hooks and caught hold of his wretched corpse. When the blessed man heard this, he ordered notwithstanding that one of his companions should swim out and bring back to him, by sailing, a boat that stood on the opposite bank.
Hearing this order of the holy and memorable man, Lugne mocu‑Min obeyed without delay, and putting off his clothes, excepting his tunic, plunged into the water. But the monster, whose appetite had earlier been not so much sated as whetted for prey, lurked in the depth of the river. Feeling the water above disturbed by Lugne's swimming, it suddenly swam up to the surface, and with gaping mouth and with great roaring rushed towards the man swimming in the middle of the stream. While all that were there, barbarians and even the brothers, were struck down with extreme terror, the blessed man, who was watching, raised his holy hand and drew the saving sign of the Cross in the empty air; and then, invoking the name of God, he commanded the savage beast, and said: "You will go no further. Do not touch the man; turn back speedily". Then, hearing this command of the Saint, the beast, as if pulled back with ropes, fled terrified in swift retreat; although it had before approached so close to Lugne as he swam that there was no more than the length of one short pole between man and beast.
Then seeing that the beast had withdrawn and that their fellow-soldier Lugne had returned to them unharmed and safe, in the boat, the brothers with great amazement glorified God in the blessed man. And also the pagan barbarians who were there at the time, impelled by the magnitude of this miracle that they themselves had seen, magnified the God of the Christians.
Source
Concerning a Certain Water Beast Driven Away by the Power of the Blessed Man's Prayer
Also at another time, when the blessed man was for a number of days in the province of the Picts, he had to cross the river Nes [Ness]. When he reached its bank, he saw a poor fellow being buried by other inhabitants; and the buriers said that, while swimming not long before, he had been seized and most savagely bitten by a water beast. Some men, going to his rescue in a wooden boat, though too late, had put out hooks and caught hold of his wretched corpse. When the blessed man heard this, he ordered notwithstanding that one of his companions should swim out and bring back to him, by sailing, a boat that stood on the opposite bank.
Hearing this order of the holy and memorable man, Lugne mocu‑Min obeyed without delay, and putting off his clothes, excepting his tunic, plunged into the water. But the monster, whose appetite had earlier been not so much sated as whetted for prey, lurked in the depth of the river. Feeling the water above disturbed by Lugne's swimming, it suddenly swam up to the surface, and with gaping mouth and with great roaring rushed towards the man swimming in the middle of the stream. While all that were there, barbarians and even the brothers, were struck down with extreme terror, the blessed man, who was watching, raised his holy hand and drew the saving sign of the Cross in the empty air; and then, invoking the name of God, he commanded the savage beast, and said: "You will go no further. Do not touch the man; turn back speedily". Then, hearing this command of the Saint, the beast, as if pulled back with ropes, fled terrified in swift retreat; although it had before approached so close to Lugne as he swam that there was no more than the length of one short pole between man and beast.
Then seeing that the beast had withdrawn and that their fellow-soldier Lugne had returned to them unharmed and safe, in the boat, the brothers with great amazement glorified God in the blessed man. And also the pagan barbarians who were there at the time, impelled by the magnitude of this miracle that they themselves had seen, magnified the God of the Christians.
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