Many years back, I remember reading a rather unique take on the Human-Turned-NMM premise. However, I've been trying to find it for a re-read recently, with no luck. I have some distinct memories about the general plot, but nothing about names, titles, or how anything would have been specifically worded. Some details might have been jumbled over time, but I don't think they were too badly. It was a pretty memorable concept.

This story begins with a cult that worships Nightmare Moon. They sought to resurrect her, I think using her old armor left behind after the Elements of Harmony purified Luna. Their ritual "succeeds", but unbeknownst to them, the soul now residing in this distinctly male body is that of a regular human from earth, not the goddess they were hoping for.

This former human, fearing what may happen if the truth is revealed, is forced to play the part of his supposed identity while taking over as the official leader. He pretends to be evil and nightmarish to appease the former leader, a nightmare-fetishist nutjob, while showing every mercy he can get away with to the rest of the members, which are pretty much all normal, sane ponies.

I remember one scene, I think the first time the main character requested breakfast, where the former leader instead sent a foal up to his designated living quarters. He had to explain that no, Nightmare Moon didn't regularly each children, rather that was only used once long ago as a threat to intimidate an enemy force. Another time, the former leader got really pissy and outright demanded that everyone in the cult be "blessed" with nightmares. The human followed through, but only for the former leader, who wasn't all the satisfied to get what he wanted after all. Surprise, surprise.

However, internal affairs weren't the protagonist's only concern. The entire reason for the cult forming at all, considering how only the leader ever subscribed to the ideology of nightmares, was that this version of Equestria had fallen to darkness. Twilight Sparkle had become infected by a nightmare of her own, banishing Celestia and Luna each to their respective celestial bodies, and becoming sole tyrant of Equestria. Nightmare Moon was promised to be a final hope to save the world, a final alicorn who could possibly combat the eternal Twilight.

Unfortunately. or perhaps fortunately for them, they got a clueless human in an alicorn body instead.

I'm pretty sure it was incomplete back when I read it, but enough time has passed that the status could be anything now. At the very least, most of what I read was centered more around building up the castle/fort/base that all the cultists lived in, guarding it against the surrounding forest creatures while the main character mostly spends time learning how to be an alicorn and an "evil" ruler. He hadn't ever revealed himself to Twilight as far as I remember, but I think he was able to talk to Celestia and/or Luna in a dream at one point.

I think the group also recruited some wandering changelings at some point? I don't remember much about that part, other than finding like, a gigantic war changeling near the end, of what was written at the time, at least.

Does anybody else remember this story or know where to find it?

I know this story, it was in my favorites but I can't find it. The Nightmare Moon was a stallion, and he had a changeling mate. The story was nearly complete, as Equestria had been rescued but the NMM character was pursuing the remnants of the Nightmare that had possessed Twilight Sparkle. I would love to find it again and re-read it.

7761000
A guy in the Hero NMM group found it. https://www.fimfiction.net/story/255419/the-substitute-demon

I miss The Substitute Demon, Reykans stories were great