This story is based on  Falling Into A Different Destiny from @harrish6.

This AU was rather beautyfull, based on darker colours, making the stars stand out even more. But sadly, balance was needed, and so Nightmare needed to destroy this universe, too. He took one last look at the peacefull landscape, before turning around and starting to look for the human.

They were in the city, seemingly on a walk. Nobody was sure how old the humans were to begin with, so Ink made something up for each AU. This specific human appeared to be around 13, making the Destroyer question why they were out at this hour. But he was used to it by now, some AUs had rather unbelievable rules. Some of them even begged him to kill him, making him question Inks sanity. He followed them in an alleyway, not that it would matter if someone caught him killing them, they would have to die, too, anyways. But Nightmare's, normally hidden, rather childish nature got a hold of him, making him curious.

It was a really dark corridor, showing similarities to the judgement hall of this world, and that put the human, known as Frisk, on edge. Most of the Sanses were worse judges than they could've ever imagined, they were blaming whoever took the role of the first fallen human for the genocide routes, when really, it was the fault of the last fallen human. The Chara of this world, however, was silently flowing besides Frisk, looking extremely desperate, but not noticing the goopy skeleton.

Five other humans were waiting in there, wearing long robes to cover there faces. They appeared to be wizards, at least it was what their magic was indicating.

" I see, you weren't lying when you said you were hunted by a spirit. " , one of them, presumably a female, said. What ?

" Yes, they're making me do horrible things ! Sometimes, they're even taking control of me and try to kill someone completely innocent ! " , Frisk exclaimed. LIAR.

" Oh, I see. It is an evil spirit then. However, you don't have to worry, we'll get rid of it in no time. " That traitor ! Delivering Chara right to some ghost hunters !

Nightmare's P.O.V

My tentacles were sharpening, Chara didn't deserve this just because of who they were ! So I reacted on instinct, remembering my past.

Flashback

Long ago, there was a tree. But it wasn't an ordinary tree, for it bore not one, but two types of fruits. The golden apples held the wishes and hopes of all living beings, while the black ones held all their fears and jealousys. This tree, also known as The Tree of Feelings, controled the emotions of the entire Multiverse. It's guardian loved all humans and monsters, but some people recieved more negative feelings than they should have. Those poor beings were willing to do everything to get one of the golden apples, since they would grant great power if eaten. One of those people, a human to be exact, was determined enough to hurt the guardian badly. But before they could die, they killed the human. Scared because of all the blood they were loosing, the guardian panicked. If they died, the tree would die, too, since they were conected. But if she prevented this by fusing with the tree, there would be no guardian to protect the apples from other rotten humans. So the guardian used the energy of the tree itself in order to create two souls, one made out of positive energy, the other made out of negative energy. Now they only needed a body for the new guardians, but when they tried to give them human bodies, they exploded, leaving a horrifying sight. So the guardian reached into the Multiverse, finding a perfect body, a skeleton named ' Sans ' . They gave the bodies to the souls, and they quickly took them. Thanks to the many natural holes of a skeleton, the energy didn't need to burst its way out. Seeing that their plan had succeeded, the guardian hurried to fuse with the tree.

Peace ruled the land for many years as the new guardians took care of the tree. The always happy one had been called Dream and the other one, simply because he was more calm, Nightmare. One made out of positive, one out of negative emotions. They were two sides of a balance and they considered themselves as brothers.

But the peace didn't last.

Soon people started to build a town around the hill with the tree, addicted by the many legends about it. While they loved Dream for protecting the golden and positive apples, they hated Nightmare for protecting the dark and negative ones. So some of them called Dream to play with them, while others went to beat up the negative guardian. Nightmare never told Dream about the actions of the villagers, in fear of getting him hurt, and simply explained his wounds by clumsiness.

One day, it was raining heavily, everybody but the twins was seeking shelter inside. But those two lived at the Tree of Feelings, so it's leaves were all protection they had. Dream was afraid of the lightning, so he clinged to his twin with all of his might. Smiling, Nightmare took of the yellow flag they putted around the tree to hide its wound, giving it to his sibling. " This flag will protect you from everything. " , he said. Those words were like a magical spell to Dream, and he wore the flag as his cape ever since. But the insults of the villagers were getting worse, they started to tell Nightmare to kill himself. Finally, he snapped. He would show that he could be good, too, that he could take care of the golden apples just as well as Dream. Sadly, everything went wrong. The negative forces weren't supposed to touch the positive apples, so they got corrupted and turned black, just like the negative ones. This corruption was spreading like a plague, turning each golden apple and drying the tree up. That was in no way what Nightmare wanted, he hurted what he was supposed to protect ! There he stood helpless as the villagers found him with a corrupted apple in his hands, and they immediately started to blame the little one. Soon they charged at him, believing that with him gone, there wouldn't be anything bad in the world anymore. Meanwhile, Dream had arrived, and he was trying to stop the angry crowd. But everyone knew, that they would get to Nightmare eventually. The negative guardian had already accepted his unavoidable death, when a voice, different from his mothers, spoke to him : " Get stronger! Use the apples ! " In his dazed and panicked state, Nightmare obeyed without a second thought. 

A loud crunching sound was heard all over the place as he bit the apple. But he didn't stop there, he couldn't. He lost control over his own movements, and his body started to consume more and more apples, until there were non left. At least,  that was what he thought, before he saw a small golden glow in Dreams hands. He would've inspected it sooner, but, even if he was back in control, a way of sudden nausea hit him. Everything was fading into black, until a sharp cyan light burned through the darkness. It was almost blinding, but it seemed to stay, even when he closed his eyes. He stumbled forwards as another wave of burning pain hitted him, it felt as if acid was cursing through his entire body. His gaze unfocused as he heard a loud cracking sound, coming directly from his skull. He could feel parts of his bones chip of randomly, leaving an unbearable pain where they disappeared. Was he falling apart ? It surely was for the better, right ? They would finally get their real happy world, without any negativity. Nightmare closed his mouth as he felt his soul melting away, he wouldn't give his torturers the pleasure of vocalised pain. They had treated him wrong for so long. Too long. He would give away everything to see them tremble in fear for once.

He had never thoght that he would die like this, maybe he would get to meet his mother in the end. But probably not. She was an angel, and he was a demon. It would be better if she didn't have to see him after he failed his job, right ? He could feel a liquid collecting at his back, but it didn't feel like blood or marrow. Something was forming on his back, and it was dragging him down. Not for long though, because suddenly, he was pushed up. And apparently, whatever helped him, was a part of his body, since he could feel it pushing against the ground. The pain had started to fade, but his back still ached, and his left eye felt as if it was on fire. But those injuries were berable, so he went to figure out why he hadn't died yet.

Standing up, Nightmare heard a loud screech. Weirdly enough, it was coming for Dream. Why was his brother frightened ? He gave him a confused look, but the positive twin was just struggeling to get up, carefully shoving the staring mass aside, so he could get a closer look on his brother.

Nightmare's P.O.V

Why did they stop charging at me ? Had they finally realised their mistakes ? Somehow, I doubted that. I watched Dream approaching me, why did it seem as if he believed that I could snap any second ?

" Dream ? What's wrong ? " , wait a second, who said that ?! That certainly wasn't my voice. Actually,  I don't think that I ever met someone with such a dark voice before. That was when I realised that the strange voice had indeed come from my own mouth. But how ? Was it because of the apples ? Sadly, the villagers decided to use my confusion to their advantage, so they charged at me once again. One of the humans went to punch me in the ribs, but something shot out from my back, lifting them into the air. As I examined this strange extra limb closer, I found out that it was a long, goopy, dripping tentacle. Apparently there were even more, since I noticed three more addictions to my spine. Speaking of which, as my hand came into view, I gasped. They weren't made out of bones anymore, but instead of a black liquid that reeked of negativity. Shocked, I stumbled back, not paying attention to the struggling human within my grasp. Which was the biggest mistake of my life. Unknowingly to me, the tentacles sharpend from my negative emotions, cutting their neck right through. Only when a loud thud was heard, I noticed my action. I murdered someone. Just like that. And the worst part was, I didn't even feel guilty about it, I couldn't.

Another sound halled through the shocked silence, Dream had bitten the last golden apple. Why ? His eyelights started to glow a bright golden, and he summoned a staff, pointing it at me.

" Who are you and where is my brother ?! Give him back, you monster ! " , usually, I would have told him that, technically, both of us were monsters in the first place, but something made me pause. Why would he think that I kidnapped myself ?

Apparently, the villagers got encouraged by Dream's little speech, since more of them ran towards me. That was their choice then, and the last and most stupid one they could have made. Quickly, I reached out with my tentacles, cutting everyone and everything in half. Dream watched in horror as everyone around him died, and chose to make a run for it. Coward. He opened a portal, hurrying through without a second glance.

Now I stood there, in an Universe filled with the corpses and the blood of my bullies, enjoying the, for once, absolute silence. What should I do now ? I decided to make a portal on my own, and suprisingly,  it worked. Even if seemed to lead into a completely white abyss, there was a huge amount of negativity coming from that area. So, curios as I was, I stepped through,  leaving the land full of twisted bodies behind.

Why was this place so white ? I'm pretty sure that everyone would go insane after staying in here for too long...