A nobleman who had several daughters placed one of them in a lax monastery where the nuns were concerned only with vanity and pleasures. Their confessor, on the other hand, was a zealous priest with a great devotion to the holy Rosary. Wishing to guide this nun into a better way of life, he ordered her to say the Rosary every day in honour of the Blessed Virgin, while meditating on the life, passion and glory of Jesus Christ.
She joyously undertook this devotion, and little by little she grew to have a repugnance for the wayward habits of her sisters in religion. She developed a love of silence and prayer, in spite of the fact that the others despised and ridiculed her and called her a fanatic. It was at this time that a holy priest, who was making the visitation of the convent, had a strange vision during his meditation: he saw a nun in her room, rapt in prayer, kneeling in front of a Lady of great beauty who was surrounded by angels. The latter had flaming spears with which they repelled a crowd of devils who wanted to come in. These evil spirits then fled to the other nuns’ rooms under the guise of vile animals.
By this vision, the priest became aware of the lamentable state of that monastery and was so upset that he thought he might die of grief. He sent for the young religious and exhorted her to persevere. As he pondered on the value of the Rosary, he decided to try and reform the Sisters by means of it. He bought a supply of beautiful rosaries and gave one to each nun, imploring them to say it every day and promising them that, if they would only say it faithfully, he would not try to force them to alter their lives. Wonderful and strange though it may seem, the nuns willingly accepted the rosaries and promised to say the prayer on that condition. Little by little, they began to give up their empty and worldly pursuits, letting silence and recollection come into their lives.
In less than a year they all asked that the monastery be reformed. The Rosary worked more changes in their hearts than the priest could have done by exhorting and commanding them.
Source: Miraculous Stories of the Rosary by Saint Louis de Montfort