Killer Has a Shack at the Bottom of a Crater

by Spencer
(L.A.)

I remember seeing a film in my youth where a group of teens/young adults are in the woods and came across some kind of crater (maybe they were up on a plateau) where a killer had a house or shack at the bottom. I've been looking for the title of this film.

One of the guys was rummaging around in the old shack and found some mushrooms or hallucinogens that he takes. He climbs back up to the plateau and is standing at the top when the killer has climbed up behind him and cuts his Achilles' heel from behind.

Unfortunately that's about all I can remember. I
thought I had found it when I saw 'The Burning' some years back, but this is a different movie.

Thanks for any help you can offer!