Ugh...

Let me clear the Moonverse stuff up. Somebody thinks that Luna turned evil and banished her sister, making her the baddie, but that is not what happened.
Sorry, it's not your fault. I should have been clearer.
Okay... Celestia and Luna became demigods after they banished Discord to stone or however it went because the power in holding three of them was too much for ordinary ponies.
Luna was jealous of her sister and could have became Nightmare Moon if Celestia hadn't went insane. She had a spell that needed a sacrifice with a buttload of magic and it would have turned Celestia into a full goddess. The only nor strong enough was Luna, and Celestia went after her, prepared to destroy her. The spell was dark magic.
Luna had to wield all six Elements and it worked, but she nearly died, the power was too great for her. It had to turn her into a full goddess, making her look like Nightmare Moon.
She changed her name in order for her to alienate herself from the past and move on. She wasn't Luna who was forced to banish her sister, she was Nightmare Mpon, who'd defeated an ancient enemy.
Hope that cleared it up.
Ow, my wrist...