She neglected her duties, with every trip into the dream realm giving more power to the darkness within her. And her failure to complete her duties to Celestia's satisfaction only lead to her sister's continued criticism. Telling her that if she wanted their subjects to love her that shed have to work for it, that their loyalty was earned through her service to them, that she was only worth what she gave to them. These comments sent Luna into a spiral.
Luna withdrew even more, spending every free moment fighting the Tantabus and it's grip on her psyche. She only gave council to her most loyal of followers, and rejected all others, something that Celestia especially disapproved of.
But Luna couldn't take it any longer. She finally released the Tantabus, allowing it to consume her. With its dark power combined with her own body, she transformed. Her normal pegasus wings were changed to leathery bat wings, her eyes were sharp and all-seeing in the darkness, and her teeth long and vicious. In her fury, she raised the moon and placed it firmly in the sky, right in front of her sisters sun. She called out to her followers, asking them to join her in the night, away from her wretched sister's blinding daylight.
Celestia was not pleased. She demanded that Luna release the skies, and bequeath her powers, for it was clear she could not control them. Luna only hissed with rage and withdrew into their castle, barring Celestia from the entrance. After it was clear to Celestia that she was not going to be able to talk Luna out of this, she wrote up her declaration of war.
Im really proud of this illustration :3 I love lunaaaa please for the love of god download this its so crusty i hate compression