This is taken from Butler’s Lives of the Saints, edited in 1956 by Frs. Thurston and Attwater.
The founder of the religious congregation known as the Hermits of Monte Vergine came of a Piedmontese family and was born at Vercelli in 1085. After the death of his parents, whom he lost in infancy, he was kindly cared for by relations, but at the age of fourteen he abandoned his home and set out as a poor pilgrim for Compostela in Spain. Not satisfied with the hardships such a journey entailed, he had two iron bands fastened round his body. How long William remained in Spain is not recorded. We hear of him next in 1106, when he was at Melfi in the Italian Basilicata, and then at Monte Solicoli, on the slopes of which he remained for two years, leading a penitential life with a hermit. To this period belongs St William's first miracle, the restoration of sight to a blind man: The cure made him famous, and to avoid being acclaimed as a wonder-worker he left the neighborhood to stay with St John of Matera. They were kindred spirits and became close friends. It was St William's intention to proceed on a pilgrimage to Jerusalem, and he would not allow himself to be deterred by John's assurance that God had other work for him to do. He actually started, but he had not got far when he was attacked by robbers. He took this as a sign that John was right, and relinquished his journey.
He now betook himself to a height between Nola and Benevento, which was then called Monte Virgiliano--possibly after the great Virgil, who is said to have sojourned there. At first William attempted to live there as a hermit, but he was soon joined by would-be disciples, both priests and laymen. He formed them into a community and from the church which he built in 1124, under the name of Our Lady, the mountain has derived its present name of Monte Vergine. The rule he instituted was most austere : no wine, meat or dairy produce was allowed, and on three days of the week only vegetables and dry bread. After the first fervor had cooled, murmurs arose and there was a general demand for relaxation. William had no desire to constrain the malcontents, though for himself any relaxation seemed unthinkable. He therefore chose a prior to rule the community, and then departed with five faithful followers. With St John of Matera, who now joined him, he made a second settlement at Monte Laceno, in Apulia. Here, however the barrenness of the soil, the exposed position and the high altitude made life misery to all but the most hardy, and even they could with difficulty hold out through the winter. St John had more than once urged removal, when a fire which destroyed their huts compelled them to descend into the valley. There the two holy men parted: John to go east and found one monastery at Pulsano on Monte Gargano, and William to found another on Monte Cognato in the Basilicata. When that community was well established St William treated it as he had treated the monastery at Monte Vergine-he gave it a prior and left it to govern itself. At Conza, in Apulia, he founded a monastery for men, and at Guglietto, near Nusco, he established two communities, one of men and the other of women. King Roger II of Naples afterwards drew him to Salerno, in order that he might have the benefit of his counsel and help. St William's beneficent influence over the monarch was, however, resented by some of the couftiers who lost no opportunity of discrediting and decrying him as a hypocrite and a humbug. With the knowledge of the king, they set a trap by sending to him, on some specious excuse, a woman of loose life, charged with the task of luring him to sin. William received her in a room at one end of which a great fire was burning ; and as soon as she began to exercise her blandishments he walked away to the fireplace, parted the glowing coals with his bare hands, and then stretched himself down at full length in the space he had cleared, inviting her to lie down with him. Her horror was only exceeded by her amazement, when he presently arose, completely unharmed. The miracle led to her conversion : she gave up her life of sin and took the veil in the convent of Venosa. As for King Roger, he continued to patronize William's foundations, and endowed other houses which he placed under the saint's control.
St William died at Guglietto on June 25, 1142. He left no written constitutions, but a code of regulations bringing the order into conformity with the Benedictine rule was drawn up by the third abbot general, Robert. The only monastery of William's foundation which exists at the present day is that of Monte Vergine. It now belongs to the Benedictine congregation of Subiaco, and has a much venerated picture of our Lady of Constantinople, to which pilgrimages are frequently made.
Legend says that William began building the church on Montevergine when his only companion and helper was a single donkey. One evening, a wolf charged from the forest and killed the donkey. William ordered the wolf to take the donkey’s place. The wolf, understanding that he had interrupted God‘s work, bowed his head, and began hauling the loads of stone.
(Adapted from https://catholicsaints.info/saint-william-of-vercelli/)