Cannibal Cabin.

A group of 20 somethings want to end the summer on a high, they take the advice from a girl they met at a festival about a secret rave deep in the Valleys. When their route is detoured, they have no choice but to venture into the unknown. Once they come across a derelict aqua park, they soon realize what they thought was their salvation turns out to be the heart of the Cannibals lair.