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They'd called it the moonheart disease, once, named after a gossamer flower that originated in a distant land. It was a plant that required careful tending to coax into vibrant silvery bloom, this far from its homeland. Otherwise it would wither before it had a chance to blossom, beautiful petals dying within the confines of a bud. The same was true of an unrequited love, they said - and all the more beautiful for the love it demanded. Despite the resources needed, the moonheart had been prized by those with coffers full enough to employ dedicated gardeners to maintain their ornate displays.
Yubel had never bothered with it, and to the few who knew of their private garden the reasons were self-evident. The constancy of their duties and the inconstancy of where those duties took them demanded hardier plants that could survive days or weeks of their caretaker's absence in the field, and the moonheart was the last thing that could survive in such conditions.
Even their king never knew of the other reason Yubel eschewed the moonheart. He never asked and Yubel never spoke of their older brother who'd coughed delicate silver moondrops amidst honey-colored lilies and pallid thistle into their shared bed in his last days. It had been a quiet but prolonged end, the boy wasting away of a broken heart as the one he loved moved far away with another, and when they entered into service at the palace Yubel had vowed never to spend so much of their own heart on a lost cause.
And indeed, when they gave their body so that strange figures could putrefy it with a dragon's blood it was only for love of one who returned Yubel's devotion without a thought. They'd met an untimely end together, but not before making their mark on history. For centuries to come, people across the twelve dimensions would remember the name of the Supreme King.
The two of them had blazed bold to the end, unlike that pitiful boy who succumbed to forlorn paleness. So why then – why, when Yubel had grown strong enough to take Martin Kanou's body in the desert, had they coughed out fragile silver petals?
Time has changed, the dimension they and Judai were born in along with the era they find themselves in. The foolish students who found Yubel feasting upon the school's food stores called the petals they coughed out Hanahaki, because the moonheart is foreign to their world if it still exists at all. The sickness grew worse when they regained their true body, and yet Judai did not know them still. Prickling rose flowers white as fire accompanied the moonheart inside Yubel as dim recognition entered his eyes for the first time, tearing their throat as they screamed in the face of Rainbow Dragon's power.
Taking the body of that boy, the one for whom Judai has traversed the dimensions again…if Judai truly loves that boy now, it should have helped. But instead jagged hawthorn cascaded out of Yubel's mouth as they taunted their beloved, whole flowers and stems first stained red with the boy's blood and then blue with Yubel's own once they were pushed out of the boy's body and into their own. Those colors, stark against the fragile white petals atop their woody thorns, were the proof of Judai's cruel affection towards those he loves.
They kept going, ever seeking the harsh balm of his attention. Judai had to see all they endured for him, how they survived the Light's searing flames on the outside and the piercing blooms on the inside all to find him again in this duel.
"Your so-called love is only a conceited delusion!" Judai declares, and soft oleander begins filling their mouth as well. In a way it's a reprieve from being torn apart from the inside, but they know well that oleander is toxic to humans. What it does to hideous undying monsters that once were human, Yubel has never been able to learn.
"Was I not supposed to be deluded?" Yubel asks, slowly, because they can scarcely open their mouth without choking on petals and thorns and still Judai watches them with his cold, bitter gaze. "I couldn't have lived with the heartache unless I felt that I was being loved. Even this sickness, the killer you call Hanahaki…this is your love, is it not Judai?"
"Are you still going on about that?" Judai's eyes narrow at them. "Right now, I'm brimming with anger at you. And now, I'm going to use the power of the Supreme King you've awakened to wipe you off the face of the universe!"
And then he makes a show of looking away, before giving them a cruel smile to rival Yubel's most masterful sneers. "Or maybe, your own delusions will do that for you. The way you are now, I wouldn't be surprised if you met your end before this duel finishes. That's a way I can protect what matters to me, too."
The duel continues. Yubel evolves into their second form, then their third. Size matters not to the moonheart disease, though, and Yubel can barely keep themselves upright by the time Neos delivers them a mighty kick via Mirage Cylinder.
Yubel is left kneeling on the ground after that attack, three heads and six mouths all heaving from pain. Their breath scatters the white and silver petals that continue to tumble from those same mouths, still tinted dragonblood-blue.
How ironic. All they can do is harm others without getting hurt themself, and here they are being torn apart from the inside out. Just a few hours ago Yubel had watched another older brother succumb to illness, a pitiful finish to what could have been a brilliant duel. Now, it seems, the moonheart has come for their own pale end.
Those silver petals, frail relics of a time and place far from here, stir into a whirlwind around the two of them. Yubel remembers...
And when the whirlwind fades, so does some of the pain. Yubel coughs again with two of their heads, and this time only blood escapes rather than that stained white.
"Yubel, are you okay?"
Down on the ground, Judai looks up at them with eyes bled dry of cruelty. But it's too late, and Yubel activates Magic Chronicle before they fade away entirely—
"I'll be going with Super Fusion!"
The moonheart blossoms into dazzling splendor.