Twilight - The soft, diffused light from the sky when the sun is below the horizon, either from daybreak to sunrise or, more commonly, from sunset to nightfall.

Twilight Sparkle was the only pony who could see Luna in the darkness.
Luna wasn't saved by the Elements. She was saved by Celestia's personal protégé.

Inspired by http://www.youtube.com/watch?v=rV_0n5gp1fE&list=PLJeC6bBkdlEzrvH0WTMgA7_Ejy-K6lqiW
[Slice Of Life] tag for two reasons: I wasn't sure of the category, and the story DOES happen in the show.