Talk:Nightmare Rarity (story arc)/@comment-26973802-20131210061153/@comment-26973802-20141104041446

That's how I see it. My headcanon goes that when Celestia and Luna fought King Sombra, he could feel Luna's resentment toward Celestia so he cast a curse to magnify the jealousy and bitterness in her, and when it got powerful enough to make her decide to overthrow Celestia, that's when it manifested into a form of its own to possess her: Nightmare Moon.