You never know when evil will strike, but you can always be prepared for it. Seeing your heroes become the villains can be a disaster, but like wounds, disasters can heal. However, when heroes became villains once again, they won't fall for the same trick twice.

The Mane 6 once fallen to the dark side after exposing themselves to some water filled with dark magic and almost destroy Ponyville in their madness. They eventually were cured and return to their senses and cleanse the evil waters from its impurity.

However, a small bit of that dark water was saved and was given again to our heroes. With the Mane 6 fallen again to madness, it is up for other heroes must save them once again.

Assuming if they can this time...