It all comes down to Mikoto's miscalculation.
He's been telling Reisi no this whole time, after all. It is supposed to be him, it needs to be him; the inevitable conclusion that makes this a suicide mission is irrelevant. It is his revenge to collect, his responsibility as the leader he has made of himself. He knows the other man doesn't like his answers - he's seen the threat of tears behind Reisi's eyes, the furious edge of true panic under the aggressive demands that Mikoto reconsider. But he saw the neat lines of Reisi's uniform and the crisp edges of the other man's ultimatums, and he saw the determination settle into dark eyes, and he thought he knew. So when the Silver King steps into view, offers a smile and the potential for vengeance, Mikoto doesn't move as fast as he should. He doesn't think there's any reason to, not with Weissman there to hold the Colorless King in place and his own loss burning radiant just under the surface of his skin. Mikoto only thinks to look at Reisi when there's a flash of blue moving at the corner of his eye, and by the time he's seen what the other intends it's too late to catch up to him.
He tries anyway. His hand comes out, flaring red-hot with his determination, and Mikoto very nearly envelops Reisi along with the other two kings in a burst of hair-trigger flame. But even the instinct of that movement can't override the surge of protectiveness in his chest, as strong as that he feels for his clansmen, and he snatches his hand back, curls his fingers in tight against his chest instead, and in that first moment of frozen panic he hears Weissman start to laugh.
It's not a boy's laugh. It's not even the Silver King's laugh. It's high, manic and screeching like nails on a chalkboard, and Mikoto feels a surge of foreboding even before he picks out the uncontrolled jerk of Weissman's features, even before the sunlight catches and glints off Reisi's unsheathed sword.
"Reisi," a voice screams, his own voice screams, "Don't," but all the force of words can't stall the motion of that shining metal. Reisi's hand closes on Weissman's shoulder as if to hold him in place, as if he's bracing himself, and Mikoto watches the boy's hand drop to his waist, snake-quick, faster than any human should be able to move. There's another flicker of light, off something in Weissman's hand instead of Reisi's; then Reisi steps forward, the motion clean and abrupt with determination, and Mikoto's vision washes out into gold for a moment.
The explosion knocks him straight off his feet, backwards several feet so he lands heavily on his back. He blinks hard around the ringing in his ears, the glow in his vision, and when his sight clears he's staring up at the crumbling shape of his Sword hovering overhead.
That's important, his hazy thoughts inform him. There is something important about his Sword, something about the irregular edges, something he needed to do. That certainty propels him to his feet before he can trust his balance, before his thoughts have fully cleared into coherency, and then he blinks, and he sees Reisi, and everything else flies away again.
He nearly falls twice, tripping over ground no longer where he expects it, on his way to the other man. Weissman is gone, presumably taking the Colorless King with him, and that should be a relief, or perhaps a burden, Mikoto no longer knows which. He wasn't supposed to be alive this long after that occurrence, he should be at the eye of the explosion instead of the fringes. But it's a blue uniform on the figure on the ground, it's Reisi's arm thrown out to the side, and when Mikoto drops to his knees and reaches out to touch the other's shoulder it's the other man that groans so weakly Mikoto doesn't even have to turn him over to know.
Reisi's hand is clenched around the handle of the knife when Mikoto gets him onto his back. It's high, too high to save, Mikoto knows without even thinking of it, even if the red spilling slow but still too-fast over Reisi's chest wasn't enough to chill all his fire into ice. The other man blinks slowly up at the intact shape of his Sword; then his mouth falls open, his throat tightens on a laugh, and Mikoto wishes it didn't sound so sincere.
"Sorry," Reisi says before he drags his gaze down to Mikoto's face, before Mikoto can even voice any kind of a judgment.
He's stolen the words from the other, sapped his protest of any fire as surely as his smile is draining Mikoto's warmth away. Mikoto gapes for a moment, reaching for the words he has been denied; then he catches up, skips right ahead to work his throat around "Why?" with enough force that he doesn't need to clarify beyond that one demand.
Reisi's smile is so warm. It's not fair that Mikoto has always had to fight for that smile before, that now it's being offered freely when there's no time left to appreciate it. "Mikoto." His voice is as bad as his smile, shaking and warm and affectionate like Mikoto has never heard him. When he blinks his eyes slide out of focus for a moment before he takes a shuddering breath and brings himself back with a visible force of will. "I didn't want to live with your blood on my hands."
"Selfish of you," Mikoto says. Reisi doesn't move to help him as he works his arm under the other man's shoulders and pulls him in to lean against his chest, but his smile lingers, and when his head falls in against Mikoto's shoulder it feels more deliberate than involuntary.
"I know." Reisi takes another stuttering breath. Mikoto can't feel his own fingers; they're numb with the horrified chill washing through him. "I didn't expect to die."
"I did," Mikoto says. "I did, you should have let me."
Reisi exhales hard, and after a moment Mikoto realizes it was intended as a laugh. "You'll have to live instead."
Mikoto doesn't have anything to say to that. There should be some sense of relief, probably; he has gained another month, another day, another hour he didn't expect to have. But all he can feel is cold and all he can hear is Reisi's irregular breathing, and even without his power coursing through his veins he can imagine his Sword crumbling like a death knell over them, the threat delayed but not destroyed. If he could he would let fire surge unchecked through his veins, take the island and their clans and the two of them into oblivion together, but the cold prevents that impulse, checks his recklessness in a way he is sure he will be grateful for later. Right now there's just Reisi pulling heavier in his arms and the warm of Reisi's blood chilling even as it soaks through his shirt.
There's another little huff, even softer than the last one. Mikoto wouldn't hear it at all if he weren't listening for it, if his every sense wasn't trained with desperate attention on Reisi. But they are, and he does, and he hears the whisper of the other man against his shirt, where his head has fallen heavy under its own weight.
"I should have kissed you." It's so faint Mikoto isn't sure he's meant to hear, or if that's just the most volume Reisi can wring from his failing air. "Last night. I should - I should have kissed you."
Mikoto stares unseeing into the distance. His ears are ringing again, there's pressure in his throat and behind his eyes. After a moment he chokes out, "You should have."
There's a sigh, a whisper of regret lost almost instantly to the wind; then the last of the tension in Reisi's body goes slack, he slumps boneless and unresisting against Mikoto's shoulder, and Mikoto doesn't have to look up to know there's only one shape above him now.