Nightmare Moon's Showdown with Princess Luna

"Have you decided yet, Luna Starlight*?"
"I'm sorry Nightmare. You're a good friend to me, and I really want to show my night to other ponies. But I cannot hurt my own sister."
"This is your final decision?"
"Yes, Nightmare. I will find another way. Surely somepony out there will listen to me."
"So be it Luna. If you will not help me, I will help myself to you."
"What... what do you mean?"
"Your anger and jealously have grown stronger, and I have grown stronger with it. I'm more powerful than you now. Once I take your powers as well, I can defeat your pathetic sister and stop her despised sun from rising."
"No... no... Nightmare, I won't let you do that!"
"No, Luna, you do not have a choice. Because Luna, I am you. We are the same pony."

This sort of concludes the mini-story I had in my head. It began with Nightmare Comforts Luna and continued with Nightmare and Luna Stargazing. This little story ends with Nightmare's personality taking control of Luna's body - hence "transforming her into a wicked mare of darkness" - sealing Luna away for one thousand years until Twilight and her friends could remove Nightmare from Luna's mind.

But this leaves one final question for you to ponder: Is this battle mental or physical?

Additional Files: No magic version.
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The Nightmare Moon and Princess Luna characters and My Little Pony: Friendship is Magic are the property of Lauren Faust and/or Hasbro and/or others.
* I have seen the name Luna Starlight used by other fan works. I like that name, so I reused it here.
Thanks to DarCowAlways, NightmareMoonS and Skylark-Torch for feedback.