Talk:Starlight Glimmer/@comment-1276314-20160329195457/@comment-1031724-20160411061447

No, that was completely unrelated to Fall of the Crystal Empire. I know that's fanmade, and couldn't fit into canon anyway because Nightmare Moon kills Sombra then and there instead of Celestia and Luna banishing him

It was hinted by the creators that an outside force was involved, and Celestia and Luna were probably oblivious to this (hence Luna's concerns about Sweetie Belle becoming the next Nightmare). Nothing has confirmed that it was Sombra, but of all the other villains in the show, he is the most powerful and his curses are confirmed to survive his banishment (making the Crystal Empire disappear). That's my headcanon, although it could just as easily turn out to be Cosmos in the movie

And again, I acknowledge that Luna became resentful on her own, I think Sombra - or whoever the "outside force" was - simply magnified her existing jealousy and bitterness so it would corrupt her, although not as a separate entity of its own since she is remorseful for what she did as Nightmare Moon, and in The Cutie Remark Nightmare Moon refers to Celestia as her sister