WMG / Jason Goes to Hell: The Final Friday

Jason Goes To Hell was an in-universe fanfic written by one of the young Crystal Lake campers.

Given the huge inconsistencies and additions to Jason's mythology never before established(Body-swapping demon Jason, a bounty hunter tracking him, the "Voorhees can only kill Voorhees" rule), it reads like someone telling a story of their own theories about the monster. By "Jason Lives" Jason was an established campfire Urban Legend with little known about him other than he's a boy who became a murderous undead creature. Given the room to fill in details, some of the kids at camp in that film took interest, especially when Jason himself showed up, and crafted a bizarre tale of the killer, even throwing in a cameo from another mythical monster said to invade people's dreams.