They're not really sure what They are, anymore.
Their consciousness is confused, disjointed memories lying just out of reach. Blades of ice, glowing blue arrows, fire and an encompassing emptiness, screaming, watching Their sister cry as she is ripped away from Them, ash, fear, blood, Ichi-nii - no, Kurosaki - screaming at Them, screaming for Them, and blood, more blood, Giselle's blood, and Their skin is dark and They cannot move on Their own, and a man - is he Their father or Their captain? - crying silent tears as he reaches out to Them, and the cackle of a mad scientist, needles and burning pain in Their veins, fighting, the clash of metal against metal, a monstrous being the size of the sky, the frost swirling around Them as Their wings give way for more power.
They remember seeing Themselves lying on the ground, blood pooling on the soft grass in the Royal Realm. But They also remember being collapsed there, looking up at Themselves. They remember dying and watching Themselves die, They remember the taunts, the truth - They had failed to protect Them, Kurosaki - Ichi-nii - had failed to protect Them. They remember the final blow, the desperate attempt to save Them, They remember flash-stepping and flinching back in fear, feeling the arrow pierce through Them, in both Their forms, passing through the one known as Toushiro to stab the one known as Karin. They feel Themselves die, and as They do, They become one.
The resulting explosion of reiatsu, as these two souls, tied together by an arrow made of the most power drenched reishi anywhere in all of the realms and fired by the most powerful Quincy in existence, merge into one, is enough to shake the very universe. The power spreads across the Royal realm, and people begin to dissolve, to become nothing. When it is over, They are the only thing left within the Realm.
The war ends with no further conflict, the Quincy surrendering after the death of their King. They don't know how They know this, but They do. The shinigami do not know what has happened, only that many of their own have failed to return. They mourn, create a monument, and move on with their lives. A young Academy student with honey brown hair kneels in front of it every night and prays, and They hear her. Kurosaki Yuzu prays to the family she has lost, the brother and sister, the father. They hear, but They do not know how to respond. They know so very little about Themselves, about what They are, and how to use the power that They have.
And so They sit there, and They listen. They watch. They feel. And the world keeps turning, the sun keeps shining, and souls continue to circle through reincarnation. As time goes by, They come to suspect, but it is not until the day that Kurosaki Yuzu stands in front of the monument, and for the first time, calls her bankai, that They realise.
They feel the brush of her bankai, of her power, and it is small. Tiny. Insignificant. Beneath Them.
They are transcendent. They are celestial.
They are the Soul King.