@ExOttoyuhr
 
@Kharn The Generally Swell Guy
 
Luna is not brash/snobbish. You’re clearly confusing her with someone else, like Trixie or Rainbow. It’s just rude.

I know I’m late to the party, but…lemme give it a go.
 
Flying is a given, obviously.
 
Shadow is…odd. I know it sounds like it would fit Luna, but given the flavor behind it, it becomes a bit more unfitting.
 
Hexproof seems out of place. It doesn’t fit in with the card’s other abilities, and in the world of Magic, Luna is no less likely to get ganked by Path to Exile than your average goblin. She’s not exactly some kind of immensely powerful being or a green creature with some kind of special natural protection. Some sort of specific protection seems more appropriate.
 
Speaking of which, protection from __ ! That’s more like it! However, black? On a blue card? ON LUNA? Black and blue are allies, and Luna is preeeeeetty susceptible to being corrupted herself. Temporary indestructible is really nice.
 
I think the biggest change that would benefit the card as a whole would be changing her color. I feel like blue was chosen because…well, Luna is blue. Luna isn’t the blue type, though. She’s not calm and contemplative. She’s brash, prone to jealousy, and very direct. If anything, she’s some sort of red combo. R/U or R/B, perhaps? I really do look forward to seeing future cards and see the decisions you make.

Well, if we’re sharing takes on Luna…
 
Luna, Moon Princess 4BBB  
Legendary Creature — Pony Avatar  
Flying, indestructible  
B: Target Pony, Pegasus, or Unicorn gains protection from black until end of turn.  
B: Target spell or permanent becomes black until end of turn.  
“Fear not the darkness, my ponies, for it holds naught more terrible than I.”  
5/5
 
Black is not necessarily evil. Just don’t ask where Luna keep her other seven and a half tails. ;)

This card feels strongly defined, but not very Luna-like. Luna’s /not/ a serene, regal, somber Princess-Goddess of the Night; she’s a loud, cocky, brash Princess-Goddess of the Night. (And an adorably cute one, too, but I think that goes without saying.)
 
So, I’d propose:
 
Luna, Diarch of the Night  
2UURR  
Flying, trample, flash, first strike  
RR: Haste until end of turn  
UU: Return Luna, Diarch of the Night to its owner’s hand.  
6/4

That’s a rather random assortment of abilities. Last I checked, Luna doesn’t exist trapped in a parallel dimension (shadow.) The green mana antisynergy is weird, unnecessary, and not at all intuitive given the flavor. And blue is black’s ally.
 
In all, it feels like you threw a bunch of abilities together and hoped it would come out moon-horse-shaped.

Pretty expensive to play. I would reduce the activated ability by one blue mana and change the mana cost of Luna to 6UU or 5UUU. Other than that, the card’s fine.