I'm particularly fond of the weapon she has in comparison to Luna. While your Luna wields a thin, almost rapier-like longsword which suits her more elegant and royal status, Nightmare Moon brandishes a large claymore which has a far more intimidating appearance. I think that kind of dramatic transformation between the two is simply genius! Very well done! Also, even if they look cold and cruel, she has really pretty eyes.
"In the beginning there were two sisters, rulers of the land. They were made living goddesses, with the elder sister controlling the sun and it's warm light, and the younger had dominion over the moon. But the younger grew jealous, and forged in secret a suit of armor that gave her power over all darkness, and waged a terrible war on her sister. In the titanic struggle, the older sister could not strike down her sibling, but banished her to the same moon she once ruled. But Nightmare Moon, as she was known, swore that in a thousand years she would return and reclaim what was hers, and exact her revenge."
Nightmare Moon and Luna don't seem too distinct outside of themes in fashion sense and hair length. The two would probably be easier to differentiate if Nightmare Moon had a mask covering half of her face, darker hair, some shadows covering her from the foreground, and, maybe, a slasher smile. Regardless, this is awesome.