So, this whole story was actually based off this one picture of a Noctis-Ardyn role switch by amidahime on tumblr. I highly suggest you check that picture out here by simply getting rid of the spaces I put there:

amidahime. tumblr post/155723225119/ardyn-caelum-doomherald-agestoryeverything

Also, a quick shout out to Racoco for co-creating and editing this story. Love ya, sis! That's all. Enjoy, everybody! :)

Scourge of the King

Prologue

Kill, kill, kill. Eliminate the threat. Infect it. Grow. Grow in strength. Not enough. Kill!

"You think ten years is a long time?" The words, the angry growl of loss and malice, we're shoved roughly up the host body's throat as the arm swung down, carving a slice through the flesh of the invader.

Speak. Feed the pain. Feed the rage.

"It is nothing to me!"

Another stroke, the threat fell on its back.

"I have lived in darkness for ages!"

The host swung wildly, the rage and fear of the Scourge taking all hold over human reason—an intellect that had saved the infection from decimation for centuries. There was no time to let the host's mind control the body, the threat must be eliminated. This was the last invader—if this other human fell, the Scourge could live—keep living for as long as the world endured, perhaps longer if it could find another of those powerful being—Astrals, the humans named them—to infect.

It could know peace if only this one, puny human would die!

The Scourge swarmed into the host's brain, blocking all cognitive functions until its control was total. No time to let the flickering soul still present in the body to drive this fight. The threat had to die, and die soon.

The Scourge slashed recklessly, frantically, straining muscles and tendons, ripping at the frail strength of the host body, driving it to new lengths of desperate effort.

But the invader, the threat, still stood. It was not eliminated. It endured, countered every sword stroke that fell against it with a grunt and a steady determination in its eyes.

The Scourge felt a flicker of fear run through the conglomeration of dark cells. The human refused to die. It was a battered human, bloodied and weak, it shouldn't have had the strength to endure, but it did! It seemed to push itself harder as the minutes dragged on—minutes that should have weakened this dangerous enemy.

It wasn't often that the Scourge paused for any reason, but for a single moment, faced with the fiery gaze of its attacker, it paused, and in that hesitation, that petrification of weakness, the threat struck back, it's arsenal of strange, glowing weapons slamming into the host's body. Each and every impact burned with a searing pain that the Scourge knew well, though it had been a number of years since it had suffered from it.

Light.

It couldn't counter through the searing agony as that dark-clad threat warped about it in dizzying patterns, night-black hair whipping across stony eyes, body fading and reappearing as glaive after glaive of the ancient kings puncturedflesh and sinew, blades digging deep, lances, axes, knives, and bolts ripping through the host body until it staggered back with the final sword impaling its stomach.

The Scourge rushed to remedy the damage and restore the host to its former state, trying to clog the flow of that vital substance—blood—before it drained away completely, but only succeeded in leeching some of its own black, miasmic fluid out of the gaping wound.

The host stumbled, the human soul wresting control of its tongue to mutter peacefully, "So, that is how you would end it..."

Time was running out. The satisfaction that the host felt now at what could only be end couldn't be tolerated. The host wanted to die, it wanted this.

Stupid, weak human. Wouldn't it rather endure, live, spread, consume? Why did human hosts never cooperate? They were the most useful of the living organisms, and yet the weakest, the most ungrateful. The Scourge had protected this human from death for ages, and now it wanted to be eliminated, heedless of its savior's needs.

A strong body was hard to find. Why must this host throw that hardy shell away so wantonly?

The Scourge felt the body sink to one knee before falling backwards, a buzz of dizziness washing through its head as its skull hit the pavement.

Desperately, the infection still tried to staunch the wound in its host's gut, but the open edges of flesh wouldn't bend to its will, and the blood, blackened with the Scourge's own cells, continued to seep out.

"Now it is over, Majesty," the host muttered to the other human softly. Its brain was its own now, the Scourge too preoccupied with the wound to stop the flow of thought and speech from the human soul. It could only listen with the ears of its host to the conversation that followed as the chill rain swept across the ruined streets and the two battered humans.

"What will you do?" the host asked, "banish the daemons and bring peace?" It seemed curious, for once without anger or hate.

The threat, the human that was supposed to be the enemy, knelt down beside the body on the ground. Its face held no malice or ill will, but the Scourge knew that to be deception. That human meant to kill them—the host and the infection. It wasn't a kind being. The Scourge knew that, but the host had been taken in.

Stupid, weak humans.

"Erase me from history once more?" the human soul asked quietly, perhaps even sadly.

Such a feeble, influential thing, emotions.

If only there existed a human without them. Emotions were ever the downfall of mankind.

The rain continued to spatter down in the heartbeat of silence that followed before the threat minutely shook its head and spoke, its voice strong, determined, but slightly choked, as if a knot of something had lodged in its throat.

"This time," the enemy said, gazing down at the body of the host, "you can rest in peace."

Its face had begun to grow fuzzy. The host's vision was starting to fail.

"Close your eyes… forevermore," the enemy human told the other, and with another ripple of panic, the Scourge realized that the host complied.

It was listening to this threat. This had to be remedied.

The Scourge rushed back to the brain, abandoning the wound for the time being, trying to regain control of the mind. It wriggled its way into the gray matter, burrowing deep inside like a worm, but the host… the host rejected it. For the first time in centuries, the host had cast the Scourge out like it was some kind of disease—like it didn't want its help. The host wanted to die. It still wanted to die. Why would it not survive!? Was it so difficult to simply continue on as all things should? The Scourge cursed the human soul inside.

It had tried for so long to squash that flicker of light that remained embedded in the body, but it would not be uprooted. It hadn't budged, but had not caused trouble before today, and so the Scourge had allowed it to endure, unwilling to waste its energy on such a small, insignificant thing.

How wrong that choice had been.

The human living inside the body ignored the Scourge's presence as if it were the insignificant one, and replied to the other human, "I will await you… in the beyond."

Its eyes slid shut, and the pieces of its spirit slipped into the realm between life and death, taking the Scourge with it.

…

Ardyn watched the swirling mix of colors flow around him like a prismatic river. How had he never noticed those hues before? The gentle shift of blue, purple, and white, the underlying hints of red and gold? They were… soothing, he supposed.

He floated, patient, peaceful in the weightless realm.

This was the end. It was finally the end. The line of Lucis was about to end, and Ardyn… Ardyn would be gone.

Erased from history once more.

It wasn't an altogether disheartening thought. He wasn't sure he wanted to be remembered. His revenge was complete, and there was nothing else he wanted. He could finally forget his brother's face and move on.

Move on where, though? When one was immortal, the thought of the afterlife didn't really pass through the mind. He had sort of assumed without prior reflection that whatever came next would be much like the Beyond—quiet, surrounded only by these floating colors.

He guessed that would be alright. Solitude wasn't so bad after spending so long rubbing shoulders with the power-hungry people of Niflheim—in fact, it would be welcome. But being alone… forever without end? It would be lonely.

Ah, well. It wasn't as if he had anyone to share eternity with, anyway. It would be better for him to be alone. Maybe people like him—those who neither knew nor trusted anyone in their lives would be doomed to spend their afterlife alone.

Except… that wasn't really what he wanted. There was still one person he would have liked to share eternity with. He sort of hoped that maybe… maybe they'd like to spend it with him as well. He thought it unlikely, given how he'd betrayed them—rebelled against a partnership that had been arranged by the Six themselves, but perhaps their compassion would allow them grace to understand why he had acted as he had.

Wishful thinking, perhaps, but wasn't it better that he had the time to do that now? He hadn't daydreamed like this in so long. He'd missed it, even if the things he dreamed were melancholy.

His thoughts cut short as another presence entered into the Beyond. He looked up with a smirk as the King of Light descended onto the plane where he waited, and he swept into an exaggerated bow as Noctis halted before him, matching his stare with a look of impassive duty. He didn't seem angry as he once had in Ardyn's presence, nor did his face betray remorse for the short straw that fate had dealt him, he only appeared ready—willing to perform the one act that he had been born to carry out.

In a strange—perhaps even a masochistic—way, that was encouraging.

Ardyn straightened, pressing his fedora securely onto his head—he had to look himself when he entered the true afterlife!—and waited, saying not a word and receiving none in return. They both knew the parts they were to play. There was no more need for idle chatter.

As his eyes raised to meet Noctis once more, however, the Scourge infecting his spirit reacted, pulling images up from his mind—images of Noctis' three friends and of King Regis. The daemonic infestation warped his vision, placing the figures around him as if to taunt him.

They'll all be there when the King passes on, that wheedling, debilitating voice that forever haunted the back of his mind whispered. He won't be alone when this ends. You will.

Ardyn shook his head, bringing a hand to his forehead as if he could press those thoughts, those illusions out of his mind as a groan of pain escaped him—one that didn't sound entirely human.

No. No, he wouldn't let the Scourge take control again. He would stay here for as long as he could. Noctis would be allowed to end him, he'd see to it.

He blinked, looking back to the young king. Yes. Noctis was alone. It had all been an illusion. It wasn't real, and he would stay here, alone though he may be, to live out the rest of eternity. This was as it should be.

Noctis raised his hand, the Ring of the Lucii gleaming on his finger, and suddenly, a barrage of panic—panic that was not his own—assaulted his senses, and he raised his hand as if to counter the death stroke.

That movement had not been his own, either. He wrestled against the control of infection, trying to lower his arm as a desperate plea slipped to his mind.

Stop it. Let it end. Let this end!

He felt the Scourge pull on the magic abilities he had been imbued with since birth, powers that, like the rest of him, had been corrupted by the infections influence, and then…

A warmth began to spread up his arm.

He looked down, and there was the Oracle, her gentle, pale hands wrapped around his wrist, her face wearing that same soft look that had graced it the day she had tried to heal him at the Altar of the Tidemother. A golden light enveloped her and the spot on his arm where her hands rested, and he felt the effects of that healing glow flow through him, weakening the Scourge even as the possessed part of him batted her image away, scattering it into billions of sun-like particles that floated away into the shifting colors of the Beyond.

The Scourge wasn't gone, but it was enough.

No longer did Ardyn try to destroy Noctis. This was his moment of freedom, his rebellion against the Star Scourge.

A peace that he had never before known settled over him as the young king readied the glaives of the Lucii, and he prepared to meet his end…

Something went wrong. Ardyn sensed it in that half second before the first glaive hit. There was a strange, nauseating, ripping sensation inside his chest, as if someone had coated his lungs with tar and was now attempting to pull it off. It stuck for a moment, robbing him of his breath, then came free, fleeing his body with a lingering feeling of desperation.

Then the weapon hit him, but he felt no pain. First one, then two, then every single one of the old glaives passed harmlessly through his body until the entire Armiger was gone, dissipating into glowing, blue shrapnel behind him.

He had just enough time to register Noctis' shocked expression before his wayward soul was slung back into his body that lay on the rainy, Insomnian road, and a peaceful, smothering blackness blanketed his mind.