Okay, experiment time: I'm gonna actually plan this story! *le gasp* This idea will probably fail miserably.
It was unclear how long the battle had been raging. Hours, days, aeons, mere seconds- given the circumstances, it was irrelevant. Both time and space were warped around that one building, a corrupted world trying in vain to shake off it's corrupter. The very stones of the hall were pounded, holding only due to the outward force of the potent auras clashing between the columns. The roar could be heard for miles, the glow lighting up the caverns like fire from the sun it had never seen. It seemed that the entire world held it's breath, ensnared by the significance of this moment: the judgement soon to pass would decide the future of the world. It was light against darkness, change against stasis, clarity against cruelty.
Neither had gone through this long trial unscathed. Perceptions blurred, the darkness creeping in- even as they fought at a faster pace than the average eye can see; either of them could collapse from sheer exhaustion at any moment. It wasn't a matter of maybe, it was only a matter of the when that would end this two-warrior war.
The pair were both at their breaking point. Sooner or later, one of them had to resign, to secede, to give in to the other's desires for the world.
Except they didn't.
When both sides are perfectly matched, it only takes a single action to tip the balance beyond the point of no return. Four moves, made in those very final moments, were all that was required to settle the battle between the two greatest powers in existence, for good.
And the first of those moves came in the form of a small flash of white. The smallest sacrifice, and a lifeline was thrown. Blade struck home, and blood flew; scattering across the shattered stones, across the fighters themselves... The second move came just as fast. A facade, a feint, a false victory.
"So... I guess that's it, huh? ... ... ... Welp. I'm going to Grillby's."
Staggering behind one of the few pillars still standing, he waited calmly to see if his observation had been correct. Sure enough, the figure soon dissolved, and dust scattered across the hall along what faint breeze curled through the broken windowpanes. Even from here, he could see the smirk on the face of his enemy- but there was nothing he could do. This battle would never have ended in his favour anyway. Instead, he put what little focus he had into his own powers, and left that forsaken place.
The demon had won.
This, of course, came as no surprise to it. Somehow, it had always known that comedian would be it's final rival. Oh, it's job was not quite done, but it would have no trouble from here on out. The "king"? The "god"? They were both petty toys, to play with, grow tired of, and throw away.
This had not been the first time the world had been rocked by the Final Battle, and the demon had no plans for it to be the last.
Crossing the threshold into the throneroom, it looked down and crushed a buttercup beneath it's shoe. I will never understand his affection for these pointless things. They die so easily... Turning it's gaze towards the pair, still deep in a heated conversation, it flipped it's knife impatiently- then had an idea. Throwing the blade at full power, it noted how the small flower's eyes widened, just before the knife went through it's head. I won't have you ruining my victory this time, "god of this world".
Asgore gasped in shock. "Why... what did that creature ever do to you, that you would be so cruel?! Surely you feel some kind of remorse? ...Or emotion of any kind?" he sighed. "Something tells me... you won't just explain this over a cup of tea. So be it then."
The demon walked over slowly, and pulled it's knife free from the shredded remains of what could once have been called "god". Turning toward the false king, it raised the blade to meet the trident-point that had only milliseconds before been rushing towards it. Despite the trident being twice their size, the weapon was mere magic, and so the demon flicked the weapon away with ease. In one swift movement, the knife came down, tearing through armour, fur and flesh alike with ease. Torn in two, the king fell. His dust scattered across the flowers, and the demon reached out for his SOUL.
And yet, the shadows in the room arose. Formed a figure. A final challenger, the ultimate denial of all that it had been working towards. Even with what it had beleived to be ultimate power, it was not the strongest. This figure, the form of a twisted and corrupted monster, spoke one sentence. The third and fourth actions that denied the destruction, sealing the worlds fate.
"No lOnGer. tHis canNoT gO on." it whispered. The twisted voice, echoing against the castle walls over and over, consumed all sound.
Then it rose, reaching for the demon, seizing it, dragging it away into the void. The room was left empty, with all that hinted to what had gone down being a battered and dead flower, the dust across the garden, and a single, bloodstained dagger.