Freedom. Something the race of humans on the planet of which Equestria stands upon along with many other mighty countries don't know. Humans use to be free and traveled far and wide but their ambitions drove them to their own descrution. They challenged the other intelligent race and they lost major wars against their magic. They than became slaves to the creatures they wish to over through. But that was over a millennium ago. Only a handfull of humans are left and they all belong to Celestia. She uses them for experiments. Some don't live... others do to only die afterwards. They want to be free once again to be rid of themselves from the hatred of the goddess.
The story revolves around a group who call them themselves the big three. They want freedom at any cost.