Blessed Thomas of St. John was well known for his sermons on the Holy Rosary, and the devil, jealous of his success, tortured him so much that he fell ill and was sick for such a long time that the doctors gave him up.
One night, when he really thought he was dying, the devil appeared to him in the most terrible form imaginable. There was a picture of our Lady near his bed; he looked at it and cried with all his heart and soul and strength, “Help me, save me, my dearest Mother.” No sooner had he said this than the picture seemed to come alive and our Lady put out her hand, took him by the arm and said, “Do not be afraid, Thomas my son, here I am and I am going to save you; get up now and go on preaching my Rosary as you used to do. I promise to shield you from your enemies.”
When our Lady said this, the devil fled and Blessed Thomas got up, finding himself in perfect health. He then thanked our Lady with tears of joy. He resumed his Rosary apostolate, and his sermons were wonderfully successful.
Source: Miraculous Stories of the Rosary by Saint Louis de Montfort