What if Vice-Principle Luna secretly* had a demon form she takes on at night called Nightmare Moon* (obviously) and was the heir* to a race of demons that only feeds on nightmares but doesn't cause them.

*Only her sister knows as she was there when Luna found that out when they were in High School.
* She can transform at any time but it's weaker during the day and she uses her form to hunt down other supernatural forces etc.
* She and her sister are descendants of the previous queen but only Luna can shift as the demonic blood is way stronger in her due it being virtually non existent in Celestia.

*grumbles about the lack of comments*

I can so imagine the conversation with Celestia about it the next school day.

*Mane 7 enters Celestia's office (Sunset's doing the talking)*

Celestia: "Yes girls?"

Sunset: "Vice Principal Luna turned into a demon (insert how many nights ago)"

Celestia: "I've already known she could do it for a long time."

Sunset: *bewildered* "For how long exactly?"

Celestia: "Since the beginning."

Sunset: "And that was when?"

Celestia: "The night after she turned 16 in high school."

Luna: *appears out of a haze of darkness in her "human form¹"* "And that same night I also find out I'm also the heir to the throne of a race of demons that feed on nightmares although they are incapable of causing nightmares.

Sci-Twi: "Are you in control when you're a demon, is the change voluntarily and do you go by Luna when your like that?."

Luna: "I am in control at all times, except for the first time it is and I go by Nightmare Moon while I'm like that."

Sunset: "Well that's rather ironic."

Celestia: "How so?"

Sunset: *gives explanation*

Celestia: "I assure you it isn't like that for us though I am slightly envious of her but am much more proud of her."

Sunset: "Why? Do you not have?"

Celestia: *sigh* "No it's much stronger in her than it's in me."

Sunset: "How much."

Celestia: "To the point where it's non-existent in me."

AJ: "So.... which side does it come from and how far?"

Luna: "Our mother's side and a thousand years."

RD: *finally processed information* " Whoa whoa whoa. Let me get this straight Canterlot High's had vice-principle who's been a demon this entire time who probably could've taken down Sunset when she turned into a demon? No offence Sunset"

Sunset: "None taken and I doubt she could've done so under my control"

Luna: "Actually I wasn't and was about to do so until she was defeated."

Any further conversation will be left to one who picks this up if someone does.

Edit: This is just my interpretation of the conversation go ahead and make your own.