The ponies of Equestria were no stranger to villainy, new villains were always attacking Equestria, trying to take their home. But other than these beings, not much in the way of violence or crime occurred in the utopia. Minor bullying, theft once in a while, maybe even the odd plagiarism. However, every single occurrence of any sort of crime was few and far between, and there was one sort of crime not even the major villains ever even considered. All they wanted was control, power. Not one would ever consider this.
The world itself sensed it, the thought entering some pony's head. The very thought shook the spirit of harmony to it's core. It searched for the source, but it couldn't find it, it had been since the ancient pony tribes that it had had to deal with this, and ponies are very protective of negative thoughts, keeping them from every pony, making the source difficult to pinpoint.
The negative force grew stronger, turning from a fleeting realization into a real possibility, the spirit began to panic, the other ponies in this world weren't ready for this. None of them would be able to do anything, and the repercussions would be too extreme. The spirit had to do something! If this pony's thought became a reality, that would realize the possibility to other minds, causing the peace to break. Equestria is not ready for that, not even the pony sisters would be able to handle it.
Then, a possibility started to form in harmony's mind, there were other places she could seek help from, the spirit would simply have to ask for help from someone who could handle it, someone who could keep the calamity from spreading very far, who could enact justice like no equestrian knew how. And the spirit had just the being in mind.