This story happened when I was a teenager to my mother, not me, but I was there to see how it affected her.
My mom, who was becoming Catholic at the time, saved up her money and flew to Minnesota to attend a Catholic conference. It was autumn and the leaves in Minnesota were changing beautifully, so on her first day there she decided to take a little nature walk. The hotel concierge told her about a nearby wilderness preserve that had walking paths, and instructed her how to get to it.
My mom is a pretty fearless person even when she is in an unfamiliar place, and since there were plenty of other people there enjoying the park she wasn't worried about walking into the woods alone. She was having a pleasant walk, but it was getting dark, and she decided to head back towards her car.
As she walked back, she noticed the forest suddenly became strangely silent, and she had an intense feeling of someone watching her threateningly. It was like she was being hunted. Feeling uneasy, she started to pray. Then something she could not see began breathing heavily in her right ear. She said it sounded like a gigantic beast that had been running hard and was panting. She could also hear heavy footsteps crushing through the grass and dried leaves next to her on the same side. In fact, she could even see the grass being parted there. When she increased her speed, the unseen creature did as well. The people she had seen in the park earlier had gone home by then, and reaching her car seemed to take forever.
Mom got ready for bed feeling quite shaken and tired when she returned to her hotel room. The next day was Sunday, so she set her alarm to wake her up in time to go to Mass in the morning. It went off at the proper time, but as she sleepily reached out to quiet it, she noticed there was a second alarm clock in the room, one she had not seen before. This alarm clock had a different time, and according to it she didn't need to get up for another hour. For some reason she chose to trust it, and went back to sleep. When she finally woke up there was no sign of the mysterious second alarm clock, and she had missed Mass.
My mom returned from Minnesota more determined than before to live as a Catholic, yet she seemed strangely altered. Her mood was troubled and solitary. I remember her spending a great deal of her time sitting in her chair by the window, praying rosaries with a pained expression in her eyes. When I would ask her what was wrong, she wouldn't tell me.
Finally after about a month of this she confided in me that ever since her experience in Minnesota she had been unable to pray properly. Her prayers had become jumbled with obscene images and vulgar thoughts. It was disgusting, and made her ashamed to attempt to pray. It was explained to her by a priest that the demon from the forest had followed her home. It was assaulting her with these visions in order to stop her from praying so that she would give up on being Catholic. The answer was to fight through the sickening imagery and pray even more. So that's why she had been spending so much time in prayer. And eventually it worked, the demon went away.