The prompt for this was "Other "true" legends in the history of Remnant other than the Maidens or the Two Brothers."
When the Two Brothers looked upon their final creation both were satisfied for the first time in their existence. Humanity was as they had intended it. The Four Gifts set them apart from all other things, and the gods saw that this was good.
However, humanity's place in the world might not have been assured for long, for the creatures of Grimm still stalked the face of Remnant, continuing the endless destruction that their god had set them on. Humanity was helpless against their onslaught and was driven to the brink of extinction. Seeing one of his creations threaten another so, the God of Darkness descended on Remnant, intending to blind the Grimm so that they could no longer menace humankind.
However, there was one among the Grimm who was possessed of human shape and speech, as well as a labyrinthine cunning that exceeded all other beings. She had no intention of allowing humanity such a respite.
The Grimm Woman approached the God of Darkness and begged him to spare her kind. She asked that the Grimm at least be allowed to see each other and the face of their beloved creator so that they would not be thrown into total isolation.
The God of Darkness relented, and instead of blinding his creations, he cast a subtler pall over their senses. The creatures would see the world hazily, all things appearing beneath their notice. The only things they could see clearly were their god and his darkness.
This was exactly as the Grimm Woman desired. She knew that a sliver of the god's dark power existed within each human, growing weaker or stronger based on the choices they made. So it was that the Grimm would attack any human whose heart was filled with negative emotions, those emotions acting as a beacon in an otherwise dull world.
Humanity was still in danger, but now it was the God of Light's turn to act. Drawing upon his great power, he created a magnificent Sphere of silver which glowed with an almighty light that was inimical to the creatures of Grimm (seeing his brother's mistake, he ensured that only the Grimm were harmed by it). The light was so powerful that if the Sphere were set upon the ground it would annihilate all the Grimm from the face of Remnant. Unwilling to destroy his brother's creation completely, the God of Light instead set the sphere high in the heavens where it shone down, weakening the Grimm so that they were no longer a threat to humanity.
The Grimm Woman was outraged by this, and immediately began scheming to see the Sphere destroyed. Donning a shawl of darkness to protect herself from the light, she traveled to the home of the Keeper of the Relics, the man chosen to protect the physical form of the gods' Four Gifts.
The Keeper was shocked to see the Grimm Woman, and nearly fled from her. But she convinced him that she meant no harm. She lamented that her kind was capable only of destruction, and wished that they might be able to live in harmony with humans. The Keeper agreed, and suggested that the Relics might be used to grant Grimm the power of the other three Gifts.
The Keeper brought out the Relics, ready to test his theory. But as soon as the opportunity presented itself, the Grimm Woman stole the Relic of Destruction. The Keeper tried to stop her, but with her shawl protecting her she overpowered him. She unleashed the Relic's power, shattering the Sphere in the sky. Though a great fragment of the Sphere remained, its light was now too weak to stop the Grimm, and they set upon humanity once more.
But all was not lost. Two fragments of the broken Sphere, each no bigger than a grain of sand, fell to Remnant below and became lodged in a young girl's eyes. The girl cried out and squeezed her eyes shut in pain. When she opened them again they glowed with a magnificent silver light that vanquished any Grimm she looked upon.
With this power, she became a defender of humanity, the first of the Silver Eyed Warriors.