It all began with a wish.
Humanity had known nothing but war and devastation, against the creatures of Grimm. Even with the existence of Dust allowing them to fight back, there was too many of the monsters. No matter how quickly humanity trained their warriors, their numbers were never enough.
And yet, humanity did not crumble. Be it luck, a higher power, or just sheer stubbornness, they fought against the tides of darkness. And no matter how bleak things looked, humanity grew strong.
With the power of Aura, the light of their souls, the human race became Hunters. A single Hunter could decimate armies and turn the tide of any battle.
And yet, humanity lost. Even with their Hunters, the less skilled were massacred when the Hunters were too far away. Worse, the Grimm fed off their emotions. The more they hated the darkness, raged against the dying of the light, despaired at the loss of their comrades, the more the Grimm fed. A single defeat, no matter how minor, could trigger a chain reaction that led to the fall of kingdoms.
But there were those who saw this, and knew the answer.
Armies didn't defeat Grimm. Heroes did.
And so they gathered, the greatest Hunters and masters of Aura. Many years were spent working on a project that could save humanity.
And in the end, they succeeded. Four Heroes were summoned from another world, each able to outpace even the strongest Heroes. And for decades, they defended the world.
The first was a bloodthirsty berserker who raged against his nature. When he fought, the very territory he defended would fight alongside him, unleashing countless weapons.
The second was a humble watcher. No Grimm could stand against him, and nothing could happen to the people around him without his knowledge.
The third was a proud demon. Though she was weaker than the others, the Grimm's own mystery was turned against them when they faced her.
The fourth was a pure, innocent soul. The Grimm, which feed off of negative emotions, were starved in his presence, and in battle, he would shield all that fought beside him.
For decades they fought with the Hunters who had summoned them, each defending a great Kingdom against the Grimm. For decades, this continued.
The berserker was the first to fall. His Master fell in battle, and unable to restrain his bloodlust without him, the warrior fought the Grimm for forty days and forty nights. Even thousands of years later, the Grimm instinctively fear the territory where he fought.
The next to die was the demon. Seeking the source of the Grimm, she and her Master left to the dark continent they came from. Though the numbers lessened, they were never seen again. Some believe she is still alive to this day, still fighting the Grimm, but none have made it far enough into the continent to confirm.
The third to fall was the watcher. When his Master died of old age, he was unable to gather the Prana he needed. Even with the entire kingdom sending him their strength, it was not enough. Weakened by this, he fell in battle a year later.
Finally, the pure knight passed away, a smile on his face. He did not fall in battle, but simply by allowing himself to fade, when he saw the legacy he and his companions had left.
For in the decades they had fought, humanity had grown. Gone was the weak race that had struggled to hold off the endless tide of darkness. The Hunters were strong, and their numbers large. Humanity could protect their land with ease, and even began to take back land from the Grimm.
And most importantly, hidden away and known only by his Master, the knight had left behind his greatest tool. Unable to fuel it in life, it was left upon a Ley Line, to gather more and more power over the course of a thousand years. Inefficient, but powerful.
And now, finally, it had finished. The Noble Phantasm of the knight had gathered the Prana it needed, and began to fulfil its purpose.
It may not have been the real thing, but even a replica 'Holy Grail' was capable of granting a miracle.
And so, seven more Heroes had been summoned. Seven more were given the reins on a Legend, and granted knowledge of what it meant. Seven more were offered the power to grant their wish, if only they would slay the other six.
And to ensure that none attempted to cheat the Grail, or undo the work that the four Heroes of old had done, an eighth was summoned. A Ruler, to keep them in line.
In a perfect world, this Ruler would have manifested in a body identical to the original, displacing the original personality until its business was done. This... Was not a perfect world.
A body with only a small difference, and a name that was too close to the original, served as a 'Catalyst,' disrupting the creation of the Demi-Servant.
And that is why, on the last day of holidays before school begun again, Jaune Arc woke up to find that he shared his body with Jeanne d'Arc.