Amazing! Every single detail is just perfect. The coloring is simply spectacular, the shading adapts to the character fantastically, the background is impressive... I donīt know how to continue. Great job!
i think its because harmony is more than one pony to have, it needs to be used by six different ponies to be used correctly. If it is wield by one pony, it will be not strong enough to chase out the envy and sadness in one's heart but to send her to the moon. That explains why celestia is not seen when nightmare moon returns at the summer sun celebration. Maybe she doesn't want her sister to be sent to the moon for another thousand years so she let the mane six to free Luna.