Nightmare Moon, Discord, Queen Chrysalis, Sombra, they all have two things in common. They're evil and they were all defeated by the six heroes of Equestria, save Discord, who's no longer evil. Together they've forged a bond of friendship to stand the test of time through and through, never to break. Nopony is closer to each other then these six mares.
Yet Twilight still feels as if she doesn't truly know them, like they're hiding something. She feels as if they each have a secret, be it dark or painful, but she doesn't know what. She has to find out, but that won't be easy, considering her friends always seem to disappear whenever she wants to talk to them.
And she's not the only one who feels this way. There are others who are out to get revenge on Twilight and her friends, those who would love nothing more then to see them all burn in the fires of 'justice'.