The uninvited guests (story, CPE level)

"I'll remember this evening for the rest of my life", he thought. He was half - lying on the back seat of his parents car and trying not to fall asleep. Images from the past few hours were flashing in his head separated from sounds, which he heard as if they were coming from behind a wall. He tried to fend them off, to relax, but they were coming like a nightmare. And he knew that this nightmare would never end.
It seemed to be an ordinary camping place in the middle of a big forest that surrounded his town. He had known it well, because he had spent a lot of time there before, so he decided that this would be an ideal place for his birthday party. He could recall all the preparations, waiting for "The Big Day", his imaginations about the party... It was the first time he was allowed to make such an event without the supervision of his parents! If he had known the truth about this place, he would never allowed himself to invite his friend there... He blamed himself for what had happened.

Everything was going well until 11 PM, when they heard someone approaching. Although he had heard rumours that some strange rituals are being held here, he never believed in them, because no one could prove it. But then he believed...
There were six of them, all dressed in animal-like skins, with claws on their hands. They seemed to be as surprised as his own friends. Clearly they hadn't expected that any strangers would dare to disturb them in whatever they were going to do to. They started to roar, make all the strange sound you associate with wild animals, and rushed towards himself and his guests. He tried to say something, but... he was too shocked. He couldn't force himself to make a sound. And then came the pain.

When he woke up, he was in ambulance, his parents sitting next to him. He was more than shocked when he heard that some of his best friends were missing, either killed or... He had to prepare himself for the struggle with the police, as they were going to pull every detail about this strange "guests" out of him...