Night swim at the lake: a light, voices, and scratching

Yesterday my best friend told me. I live in the south, we have this one beautiful blue lake with high steep slopes down to the water, half an hour's drive from the city. And my friend's family has a dacha near it. So one time, when he was about 18, he, a guy named Gena, and a girl Katya, went to that dacha. And they decided to go to the lake at night to swim. So they come up to the lake and see that it's glowing a little in the dark. This of course surprised them extremely, they started moving along the path to the beach, which goes along the tops of the slopes, meaning from there you can see the beach from above. And they see that on the beach a blue light is flying. White sand, and above it a little light is flying in the air. At this point they got really creeped out, and they turned back to the dacha. And as it later turned out, each of them heard the others saying "Let's go swimming", even though each of them was actually saying "Let's go back", meaning it was like some kind of delusion came over them. Well, they're walking back, and suddenly behind them, by their sense of it, about 100 meters away, someone started calling them by name (and only the guys, the voice didn't call the girl). A voice like a 50-year-old thug calling. And at the same time, around them they started hearing voices on the sides of the road and behind, like a group of people was accompanying them. And it was impossible to understand what they were talking about, meaning chatter and mumbling could be heard, but the words couldn't be made out. Well, after that they took off running. They ran to the utility shed, locked themselves in. And then it was like someone was scratching with claws on the walls of the shed outside all around. After that my friend didn't spend the night there for a couple of years out of terror.