Create a 30-second vertical YouTube Shorts video with satisfying slime ASMR in the background. The AI voice should tell this mysterious story: "One night, a boy was playing on his phone when he received a message from his own number. It said: Don't look behind you. He thought it was a joke, so he ignored it. A few seconds later, another message appeared: I warned you. Slowly, he turned around... and saw himself standing in the doorway. His phone then received one final message: You shouldn't have turned around." Use a realistic, slightly creepy AI voice. Add large, easy-to-read subtitles synchronized with the narration. Start with a strong hook in the first 2 seconds. Keep the ASMR background satisfying and visually interesting throughout the entire video. Make the ending dramatic and surprising.