Going with the "Mysterious creatures are attacking Ponyville."

Deep within the kingdom of Equestria, magic has failed us. Something snapped within one of the most powerful ponies alive without warning or reason, and now the corruptive power has been cast, only so much time remains.
Twilight was doing what she did most every day, hanging out with her friends and getting into trouble and having grand adventures. Then Cheerile goes missing. The whole town looks for her, searching far and wide for the missing school teacher until something starts to happen. Ponies begin to disappear.

The Nightmare has started.

Silence reigns.