Run!

 It was in 2009. On an autumn day, I was sitting at home, reading websites on the Internet. Then my friend Anton called me and invited me to go with him to an abandoned nine-story building on the outskirts of the city, to climb, as we like to do. Having agreed to meet at the place at seven o'clock in the evening, I continued to sit at the computer.

In the evening, at the appointed time, I went to that building and met Anton there. The building, by the way, was built a long time ago and looks quite depressing from the outside, even from the inside. We chatted for a while, walked through the corridors of the nine-storey building, then climbed onto the roof. When we got tired of it, we started going downstairs. By that time, I noticed that my friend was behaving strangely - he was usually energetic and cheerful, but that evening he was unusually quiet, as if he was constantly thinking about something.

As we were going down, my phone rang.

"Hello?"

— Hello!  Anton's voice came over the speaker. — Sorry, Serega, I was with my mom on business, I was terribly late, so I'm not going...

I turned to my friend in complete confusion. Anton, who was standing next to me, looked at me and grinned. The grin on his face (whoever he was) was so scary that I literally fell into a stupor.

My memory has not preserved what happened after that. The only thing I remember is that the creature said one word to me in a thin, squeaky voice.:

"Run!"

I don't remember how I got home. 

To this day, I can't figure out what it was.