After Nightmare Moon returned from her 1000 year banishment, the main 6 didn't know that the monsters hidden under Canterlot's mountain became free and began settling in Canterlot and seceretly...PonyVille
5 years later, the main 6 started to become annoyed of the monsters living in there lives and decided to put an end to them.
3 month after there successful 'clean up' nearly everypony forgot that they even existed, that was until a curtain royal guard arrived in PonyVille, and what the main 6 don't know is that they might see WHAT DEATH IS REALLY LIKE!!!!