Saint Dominic had a cousin named Don Perez or Pedro, who was leading a highly immoral life. When he heard that his cousin was preaching on the wonders of the Rosary and learned that several people had been converted and had amended their lives by means of it, he said, "I had given up all hope of being saved but now I am beginning to take heart again. I really must hear this man of God."
So one day he went to hear one of Saint Dominic's sermons. When the latter caught sight of him, he struck out against sin more zealously than ever before, and from the depths of his heart he besought God to enlighten his cousin and let him see what a deplorable state his soul was in.
At first, Don Perez was somewhat alarmed, but he still did not resolve to change his ways. He came once more to hear the Saint preach and his cousin, realizing that a heart as hardened as his could only be moved by something extraordinary, cried out with a loud voice, "Lord Jesus, grant that this whole congregation may see the state of the man who has just come into your house."
Then everyone suddenly saw that Don Perez was completely surrounded by a band of devils in the form of hideous beasts, who were holding him in great iron chains. People fled in all directions in abject terror, and Don Perez himself was even more appalled when he saw how everyone shunned him. Saint Dominic told them all to stand still and said to his cousin, "Unhappy man that you are, acknowledge the deplorable state you are in and throw yourself at our Lady's feet. Take this rosary, say it with devotion and with true sorrow for all your sins, and make a resolution to amend your life."
Don Perez knelt down and said the Rosary; he then felt the desire to make his confession, which he did with heartfelt contrition. Saint Dominic ordered him to say the Rosary every day; he promised to do this and he entered his own name in the register of the Confraternity. When he left the church his face was no longer horrible to behold but shining like that of an angel. Thereafter he persevered in devotion to the Rosary, led a well-ordered life and died a happy death.
- from The Secret of the Rosary, bySaint Louis-Marie Grignion de Montfort