The Mare of Madness has been trapped in a prison for thousands year by Princess Celestia. She has trapped her in a dimension that seals her powers from Equestria. The seal was created with the Elements of Harmony, but the effects have been wearing out and the madness that she sends out has been effecting Twilight, who just turned into an alicorn. The madness has been giving Twilight nightmare and she can also see them when she is awake. The Mare calms herself as Princess Celestia's other sister, but her being imprisoned no pony knows about her. So Twilight has to seal her powers again by going to the dimension that where the Mare of Madness resides. Hopefully the madness doesn't take her over.