Reincarnated

A huge novel fan dies and reincarnates as a character in a novel he loved to read. The character he becomes is the ghost that haunts Nightmare and manipulates him into giving up control over his body becoming the most feared man in the entire multiverse and known as the king of negativity. Jason figures telling Nightmare exactly what will happen to him if he ever eats the apples he protects will prevent this future from ever occurring not wanting Nightmares blood on his hands when he could have prevented it.
This doesn't go as planned.

Explanation:
The undertale multiverse is a story that Jason the character read while he was alive before things hit the fan and he became corrupted Nightmare. He doesn't know all the characters backstories just Nightmares because the book follows Dream and his adventures. Dream will continually mistake Jason for his brother throughout the story eventually getting original Nightmare to convince him this is technically true because he came from the tree like they did making Nim his mom as well. Just letting you know in case there is any confusion with the description.