Where Every Scroll is a New Adventure
Two fun (separate) ideas for Nightmare that have been festering in my mind for the past bit.
1. Nightmare has every memory of his past self as a child. He firmly believes that the real Nightmare died and that he's just the corruption. Every single day, he questions if having the exact same memories makes him a copy of the same person that never got to grow up. (This could go in either the "nightmare never died" or the "nightmare died, but corrupted nightmare is still valid" type of existential routes. Either way, fun!)
2. Nightmare has DID. The corruption actually did nothing to cause this; it was all the villager's abuse. He only noticed this after the corruption though, and he blames it on that. (Maybe, one day, something causes him to have a third alter, and suddenly the first two are spiraling as they try to figure out what just happened)