Door Number X0

When I returned home from studying abroad, I noticed an 'X' written next to my front door. I didn't pay attention and simply wiped it off. But when I returned home that day, the 'X' reappeared. I thought it might be some kid playing a prank. That night, I was working at home when loud cartoon noises came from the neighbor's apartment. I was too annoyed to work, so I knocked on the neighbor's door to discuss it. The next day, when I left for work, the 'X' on my door had turned into a '0'. Just as I was wondering about it, the neighbor's door suddenly opened, and when I saw the neighbor, I knew danger was coming...