After his best friend was used as a human sacrifice to protect a town from a mythical beast, a telepathic teenager must stop the cult from choosing his girlfriend next.

I don’t think you need the first bit, that seems like backstory as it is the selection of his girlfriend that sets the story in motion.
So, my version would be something like: When his girlfriend is selected to be sacrificed to please a mythical beast, a telepathic teen must return with the head of the beast before she loses her’s.

It’s 27 words and a hit rough, but think it has a clearer goal, a ticking clock etc.

As said you can throw the best mate bit in the setup section of the screenplay.