When a Monster Knocks on the Door

After a busy day, Master Fang just wants to dive into the soft and warm bed and have a good rest when he comes home at night. He lay on the quilt, feeling extremely relaxed. At this moment, there was a sudden knock on the apartment door. "Dong dong dong." A crisp knock on the door echoed in the quiet apartment. But he was the only one staying on this entire floor. Who would knock on the door?