St. Placidus, disciple of St. Benedict, the son of the patrician Tertullus, was brought as a child to St. Benedict at Sublaqueum (Subiaco) and dedicated to God as provided for in chapter 69 of St. Benedict’s Rule. Here too occurred the incident related by St. Gregory (Dialogues, II, vii) of his rescue from drowning when his fellow monk, Maurus, at St. Benedict’s order ran across the surface of the lake below the monastery and drew Placidus safely to shore.
It appears certain that he accompanied St. Benedict when, about 529, he removed to Monte Cassino, which was said to have been made over to him by the father of Placidus. Of his later life nothing is known, but in an ancient psalterium at Vallombrosa his name is found in the Litany of the Saints placed among the confessors immediately after those of St. Benedict and St. Maurus; the same occurs in Codex CLV at Subiaco, attributed to the ninth century (see Baumer, “Johannes Mabillon”, p. 199, n. 2).
There seems now to be no doubt that the “Passio S. Placidi”, purporting to be written by one Gordianus, a servant of the saint, on the strength of which he is usually described as abbot and martyr, is really the work of Peter the Deacon, a monk of Monte Cassino in the twelfth century (see Delehaye, op. cit. infra). The writer seems to have begun by confusing St. Placidus with the earlier Placitus, who, with Euticius and thirty companions, was martyred in Sicily under Diocletian, their feast occurring in the earlier martyrologies on 5 October. Having thus made St. Placidus a martyr, he proceeds to account for this by attributing his martyrdom to Saracen invaders from Spain — an utter anachronism in the sixth century but quite a possible blunder if the “Acta” were composed after the Moslem invasions of Sicily. The whole question is discussed by the Bollandists (infra).
From the Catholic Encyclopedia