Legend of the Nether Knight

When Grandpa Li was a child, he and his brother were playing in bed one night. Suddenly they heard the sound of walking at the door, rustling. Grandpa Li and his brother shouted at the same time, "Mom, Mom, Dad is back," and Mom also ran past with a smile on her face. I went and opened the door and found that there was no one at the door. I thought I heard it wrong, closed the door and continued playing. After half a quarter of an hour, there was the sound of walking at the door again, which was also rustling. Uncle Li and his brother shouted at the same time, "Mom, Mom" Mom, dad is back. Mom walked over, opened the door, and found that there was still no one at the door. Mom looked at the door again to see if it was the sound made by other things. There was nothing but silence. Mom came back muttering, and Uncle Li and his brother continued to play. After another half-quarter of an hour, there was the sound of walking at the door again, and it was still rustling. Uncle Li said that he and his brother didn't know why, and they suddenly became nervous. Mom walked to the door.