As the title says I came up with an idea of my oc character Telwyn Freecolt a male blank flank earth pony in the profile picture transforming into the anthro version of Nightmare Moon. (Minus his hazard suite)

https://e621.net/posts/241651

The link picture of a sexy Nightmare Moon is what my character Telwyn Freecolt ends up looking like.
Now I have a description of who Telwyn Freecolt is and why he is gets transformed into Nightmare Moon.

Telwyn Freecolt is a young adult male earth pony, he is a collector of sorts a bit of a Dr too he lives in a old castle that overlooks a pony town maybe Ponyvilla.
Telwyn dose not show his emotions unless he feels he must. He always show a poker face the look he has in the profile picture of mine is the example. His staring makes other ponies in uncomfortable.
Telwyn past is mostly unknown as his blank stare gives nothing showing no signs of abuse, or joy as he never talks of his past.
Telwyn has no cutie mark when I created him I could never think of a cutie mark for him so I decided to keep him a blank flank and made the idea he just never for some reason found what his cutie mark could be.
Telwyn is seen a bit insane and mad by other ponies whom think Telwyn is a crazy mad Dr and their right as he is obsessed with collecting anything of interest to him and over his research of becoming something more then a simple earth pony.
Telwyn Freecolt researches for ways to become immortal. Telwyn is very curious about the pony body and studies test subjects he pays so he can learn how to make himself immortal.
Telwyn Freecolt may keep one pony around as somepony that he pays money, but he is also Telwyn's test subject or a slave in better words.
Telwyn is sort of like a Dr Frankenstein.

Telwyn could be a worshipper of Nightmare Moon for whatever reason.
Telwyn may love and adore Nightmare Moon.
Telwyn Freecolt soul for some odd reason is close and related to Nightmare Moon spirit.
Nightmare Moon spirit lives on and follows Telwyn Freecolt if Nightmare Moon spirit is real to begin with.

What ever the case Telwyn enjoys collecting many old things ranging from weapons, crystals, armor, bloom of history and races, pictures and art, some of which is Nightmare Moon's armor and clothes she wore as seen in the picture in the link. One of Telwyn Freecolts favorite quotes. "The pearl of my collection." A quote he says whenever he has collected something new or some pony meets and sees his collection and asks him of it.
Telwyn Freecolt is also a fan of Nightmare Moon related things like art and pictures of her. He is willing to pay for anything related or belonging to her prior or after her defeat.
Telwyn found all of Nightmare Moon armor and clothes from where she was beaten. And took it all back to his castle and with his collection.

Now I have a few ideas on how and why he gets transformed into Nightmare Moon.
One is if Nightmare Moon's spirit dose not exist. Telwyn Freecolt one night ends up deciding to put on the armor and clothes of Nightmare Moon which are unknown to Telwyn magical. Once Telwyn puts them on he gets transformed into the anthro version of Nightmare Moon in the link above with control over his mind and his female unicorn body and powers l, and knowledge, and knows how to fly, use magic, and even fight, and sounds very much like Nightmare Moon. And Telwyn with Nightmare Moon body has her cutie mark too and is no longer a blank flank.

2 Nightmare Moon spirit exists, then Nightmare Moon is surprised but happy she has an admirer keeping her legacy alive even if Telwyn is a blank flank. Nightmare Moon notices how Telwyn Freecolt soul is close and connected to her own soul.
Nightmare Moon sways Telwyn Freecolts mind and communicates with him, learning about Telwyn and his past. Then one night Nightmare Moon then seeking to gain a new body enthralls Telwyn to put on her armor and clothes and once Telwyn dose she merges her spirit with Telwyn turning Telwyn into the anthro version of Nightmare Moon. The draw back is that Nightmare Moon did not expect is Telwyn remains in control of his female sexy alicorn body with Nightmare Moon and Telwyn in a sense two as one personality fused together.
Telwyn will now speak in third person a little more and say we instead of I on account Telwyn and Nightmare Moon are fused. Both Telwyn and Nightmare Moon speak at the same time.
What happens after the transformation I can give in a pm. if someone wants too make this into a story by all means go head I write it but I'm already writing a story already I have so many ideas but I'm not able to use them all.