WMG / Worth the Candle

The Dungeon Master is Fel Seed.

A kid named Arthur Blum is dying or comatose, and somehow created a whole fantasy world to inhabit, based on his idea of the "Juniper Smith cinematic universe:" a giant crossover of all of his friend's settings. He doesn't understand why he's there, and over decades came to mistakenly believe in a Dungeon Master controlling everything, and see narrative structure when all there is is some principle keeping him alive. Something emerged in, or entered the realm—perhaps a seed fell in and is growing into a weed, a parasite that feeds on these psychically-created universes. Because of its outsider status, like Arthur it can bend, but not break, the rules, especially the Exclusionary Principle. So while it was trapped in an Exclusion Zone, its influence still extends outside of the Zone and it plots continually to escape. Arthur went on a quest to defeat it for good, and ended up stealing the key to its escape, but was trapped in a tunnel beneath its realm; if he leaves, Fel Seed will take the key back and doom the world. Fel Seed keeps trying to trick him into leaving, and over the centuries it developed its most elaborate scheme yet: creating a fake Juniper Smith who thinks he's real. Juniper is a construct drawn from Arthur's memories, and doesn't necessarily correspond to the real Earth (Arthur's memories: "I bet Juniper kept putting all this SJW stuff in to impress Tiff so that they could secretly date." Fel Seed: "Seems fake, but okay."). It's guiding him through a series of adventures meant to make it seem plausible to Arthur, who can see some of what's happening through his own entads, that a godlike version of his friend will eventually appear, knowing Fel Seed's weakness. The Dungeon Master is another manifestation of Fel Seed, and is bluffing about a lot of his claims to omnipotence (he doesn't control the exclusionary principle, just knows when something has been excluded, for example). Right before it's too late, Joon and Arthur will figure this out, and Joon will turn against his creator.

---