Princess Celestia entrusted Twilight with much more than the reformation of Discord.
She also entrusted Twilight to be careful with her search into the missing god of chaos.
Locked away for centuries, the Reality Cipher was something so dangerous that supposedly not even Discord himself dared to touch it again. Twilight, however, was very eager to discover the knowledge lurking within the ancient god's peculiar secrets.
Perhaps, a little too eager.
Once the unraveling begins, it's impossible to stop.