AU idea is originally made by VictoriaLopez658! They suggested a concept to me in one of the comment sections :)

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[Sabre's POV]

.

..

...

...Hm...?

...What? Where...

Where am I?

I open my eyes to the sight of some kind of... lab. I'm inside something that looks like a giant test tube. My mind is suddenly torn away from looking at my surroundings by an overwhelming sense of pain, as I quickly shut my eyes again. Everything stung, yet... also burned...

It hurts... like I was thrown into lava after being electrocuted... I know it feels like I've been electrocuted because there's a tingly sensation in my hands, familiar to me because of how many times I've accidentally been caught in the crossfire of fights between two Steves. The water in the tube I'm inside kind of helps with that, and the weird burning.

I slowly open my eyes once more, minding the terrible sore feeling all over my body. I try to look around, and I see a bunch of brewing stands on tables, some books with quills strewn about near them, dark grey carpet over a cracked stone brick floor, redstone scattered around it, charts hung up on walls the same material as the flooring, and a doorway in front of me. I see a hall past it, faded red carpet making a path through it, but I can't see into any other rooms.

The burning feeling gets stronger, while the static-like one fades, and I look at my hands. They're covered in what looks like large, smoldering burn marks, and there are a different kind of burns next to those, only peeking out from my sleeves... they look a little like lightning bolts etched into my skin. The only thing that's the same about the two types of burns is they hurt like heck, and they're where the feeling of burning alive comes from. I also notice while looking at my hands, that my shirt is ashen and torn up, my sleeves ripped at the ends.

As I try to move around, I feel that it's not only my hands that are burning again, but my entire body. I must have more of the burns all around me. I then notice the reflection of my face in the glass, and I see that I have more of those burns there, too; only one larger burn on the side of my cheek, while a ton of the lightning-esque burns cover my forehead. 

...Why am I here? Why am I covered in burns? The last thing I remember is I was walking with Light while heading to meet back with Rainbow and the others, after checking for Darkness around the Yellow outpost, and... now I'm here, I think. Did someone take me? Am I stuck with someone that's an ally... or an enemy?

My thoughts are interrupted as I hear a mechanical noise around me, then some kind of dark blue stuff enters the test tube I'm in, floating in the water like a handful of dirt thrown into a pool. Pretty soon, I can't see anymore. 

Suddenly, I... I feel... tired...

...

..

.

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I jolt awake, suddenly sitting up in a grey bed, inside what looks to be a prison cell. There's a metal cuff around my wrist, connected to a chain that leads into the wall next to me. Yep, I'm definitely captured. How did I end up in a cell, though? Last thing I remember is being in some kind of test tube, covered in burns- wait.

I check my hands again, and there's no longer any of the burns that were there. I'm not in pain anymore either, so any other burns I had must be gone too.

...This is so weird. What am I doing here? Why was I burned? Who fixed it? Am I even anywhere near home? What is happening, dang it?!

"Ah, good to see you're awake."

I turn my head to see who just spoke to me, and I'm filled with both fear and even more confusion.

Standing in front of me, is Nightmare Steve. He's watching me from the other side of the cell bars. Now, it looks like more Darkness is covering him, as the splotches he previously had have turned to large, dark spots covering his torso and some of his arms. His previously neon red eyes have turned more to the color of blood.

"Wh- How- You're supposed to be dead!" I scream as I start backing into the wall, still on the bed.

"Maybe, but I simply could not accept that my time was up just yet." Nightmare grins.

I stay silent, and watch as Nightmare stares back at me.

"...What do you want with me?" I ask, trying my best to give a steely glare to hide my fear.

"Well, it's not really a question of what I want, now." Nightmare says, tilting his head. "But rather, what you want. I was just ordered to revive you."

"What?" I say, shocked. "Revive me?! But I didn't-"

"Die?" Nightmare interrupts. "Hm, it seems you don't remember what happened... allow me to jog your memory a little bit."

Nightmare raises his hand, a dark glow appearing, and suddenly I'm seeing Light Steve in front of me. We're in the middle of the forest, arguing, but I can't hear what's being said. Light looks angry at me, and I seem like I'm trying to defend myself about something, based on the hand signals being thrown around. Light stops yelling, however I'm still talking. I notice Light looking angrier and angrier, and suddenly he lashes out at me as I step back in surprise.

Within a second, I'm surrounded by lightning. It goes on for a few minutes. Then it stops, and I collapse to the floor as Light forms a look of guilt, and... fear. He runs to my side, and I'm able to hear his voice, drowned-out like he's a million miles away. He keeps repeating 'I'm sorry' and 'Please' over and over again. The corners of my eyes have a white haze, and it's creeping over more and more of my vision as a dull ringing sound gets louder, and I'm completely unable to hear Light again. The rest of my vision slowly fades to white, and...

I'm back in the cell, Nightmare watching me with a blank expression as he lowers his hand. Tears are streaming down my face, my blindfold unable to keep them at bay. For a while, all I can do is silently cry as I register what Nightmare just showed me. I then snap out of it, and I'm sent into panic.

"Y-you're lying! It's not real!" I scream. "I know you, you're just trying to trick me! There's no way that could've happened!"

"Deny it all you want, but you'll come to your senses soon enough." Nightmare says. He starts to walk away, but stops for a second. "Let me leave you with this; If this all was a trick, then why were you covered in burns only acquired from lightning? How else could you have ended up here, without any of your 'friends' already arriving to save you?"

Nightmare leaves somewhere, and I try to make sense of all this.

No, it can't be.

But why else?

No, no! He might have done it!

If he did, he would've just killed me again.

But Light wouldn't do that!

Yet I saw it for myself.

It might've been an illusion!

Nightmare doesn't have that kind of power, he closest thing he can do is show memories.

It must be made up! It can't be real!

But Nightmare has a point; why else would I have had such burns? There's no other reason.

This can't be happening!

But it is.

No...

This is reality.

It can't...

This is happening.

It can't, it just can't...

Light killed me.

No...

Light murdered me.

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I've been sitting here, staring at the wall for what feels like hours.

This... this is real. Nightmare was... he was telling me the truth. There's no other explanation.

When I finally realized it, it was like my world suddenly just... broke. Everything I've ever known here is flipped on its head. Light had killed me the same way Nightmare did all that time ago, and now Nightmare is the one that saved me.

I hear footsteps coming towards me, and I lift my head off my knees to look toward outside my cell. Nightmare's come back, of course. He stops in front of the cell door and looks at me, my tear-stained face staring blankly back at him.

"So, have you come to terms with what's occurred yet?" He asks, still having the same blank expression.

"...Yes..." I mumble, shifting my gaze to the floor.

"Are you going to try to escape when I open this door, or will you comply?" He asks me, his hand holding a key ring with two keys on it.

"Why would I? I have nothing left." I mumble again, my voice monotone.

Nightmare puts the key into place, unlocks the door, and enters my cell. He walks over next to me, and grabs the chain connected to my wrist. I was was expecting him to pull me to the ground or something, but instead he uses it to lift my arm so he can reach the shackle, and uses the other key to unlock that.

"Safety precaution." He says.

I pull my arm back to me and rub my wrist, it now sore from the shackle being around it so long. I look up at Nightmare, both confused and still uncaring. Nightmare then leans against the wall, staring at me.

"I believe you have some questions, regarding this." He says. "Now is your time to ask them."

I think for a minute, then return the eye contact.

"...What did you mean when you said 'you were ordered to revive me'?" I say.

"Since the Yellow Kingdom failed me, I now follow the Darkness entirely." Nightmare answers. "The Darkness' leader wants you on our side, so given my experience with... cheating death, he ordered me to revive you. It was simple enough, as I only had to pull your soul out of the path to the Middle World. Your body reformed by itself once I brought your soul here."

"...Why was I in a test tube?"

"You were still injured from your death, so I put you there for your body to heal, as I'm quite sure Void wouldn't want that if you're going to join us."

"Who?" I say, confused.

"The previously mentioned leader of the Darkness- Its very creator. He wants you, the chosen catalyst from the Prophecy."

I think again for a moment.

"How long has it been since I... died?" I ask Nightmare.

"Two weeks." He replied. "The cowards you called friends didn't even bother with a grave, heh... Well, I suppose they're still grieving."

I think of one last question, the one I've been kind of afraid to ask.

"...What do you mean, 'join you'?" I ask, unsure about hearing the answer.

"Ah, I was waiting for you to ask that." Nightmare grins. "I've already explained that the Darkness' leader wants you to join us. However, I have reason to believe that you'll want to join for... your own purposes."

"Such as?" I say, lifting my head up again.

"Well, you remember what happened now." Nightmare says, shifting from the wall and taking a few steps toward me. "Light Steve, one of your closest friends and the being supposed to represent the good of this world, destroyed you in cold blood. Stabbed you in the back, over a silly little disagreement."

As I reflect on what happened as Nightmare talks, I feel something replacing the sorrow inside me. Bubbling up into something I've never felt before while in this world. Nightmare continues, noticing this.

"And the funny thing is, nobody questioned him after he showed up without you. Do you know what his excuse was? He said you just disappeared when he got separated from you, after Darkness supposedly jumped out at the two of you. What a pathetic lie, right? He's made everyone look for you, even though he's thinking you've now begun rotting away in the spirit world."

...What?

Light... he murdered me, and blamed it on Darkness, just to save his own skin?

I shift off the bed and stand up, facing Nightmare. A quiet rage fills me, a kind I've never felt here.

"It seems you agree." Nightmare chuckles.

"What are you offering, Nightmare?" I ask.

Nightmare then displays a wicked smile.

"It's simple, Sabre." He says. "How about you join me, and the Darkness, and we give you what you need to take your revenge on your so-called friends. Power. An army of your own, maybe. Who knows, you could even be my new right hand. Void has already taken quite the interest in you, there's no doubt he'll give you all you need for your new life... alongside us. You will never be tossed aside or ignored by anyone, ever again. You will be respected, and accepted, for who you are meant to be.

Nightmare then extends his hand, offering his deal for me to decide if I'll take it.

"What do you say, Sabre?" Nightmare says. "Will you unite with the Darkness, take your place in our ranks, and aid in our goals?"

I think for a moment, possibilities wandering in my thoughts. Revenge against Light, using the very bane of his existence. Seeing the faces of everyone, as they gaze upon what they have done to me. Them finally remembering that their actions have consequences. I'll have hundreds, maybe even thousands to back me up. I won't be alone.

I'll be accepted as I am, as what I could be.

The dried remains of my tears crack slightly as I copy Nightmare's grin, and take his hand. Shaking it, I'm ready to leave behind all I have built, and start anew on the side of the Darkness. Nightmare releases my hand, and places his on my shoulder. A sinister smile covering his face, he tells me:

"Welcome to the winning side."

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[ A Month Later }

Nightmare is sitting atop a throne in a blank abyss, surrounded by dark pillars and fire, a blood-red carpet leading to him in front of him. Standing there is Rainbow, Light, and their new ally Galaxy Steve, who is apparently very powerful. I'm standing behind Nightmare's throne, waiting for my signal to show myself in my new alignment. As Nightmare is wrapping up his monologue, I know it's almost time.

"You see, Rainbow Steve, I have someone... more of interest to you than your new little "friend" here." Nightmare says. "Someone you know very well."

"Like that's going to change anything, Nightmare!" I hear Rainbow yell. I've never heard him raise his voice at someone before. "We're here to destroy you, once and for all, and there's nothing you can do to stop it!"

"This guy seems really full of himself." I hear a new voice say, that's most likely Galaxy.

"Nothing I can do, hm?" Nightmare says, ignoring Galaxy's comment. "I believe you'll retract that statement when you meet my new ally."

"Who did you manipulate this time, you stain of this Realm?" Light says, venom in his words.

Nightmare laughs, and I hear the signal.

"Why don't you see for yourself?"

Now.

I step out from behind Nightmare's throne, and I see Rainbow's face twist into horrified shock. Galaxy looks indifferent about me. Light, however... he has a look of pure fear. As soon as I see him, I make sure to slightly scowl.

"S-Sabre..." Rainbow whimpers.

"Sabre? The guy you told me about?" Galaxy asks, looking clueless.

"Yes, indeed. Sabre has joined the side of the Darkness, thanks to a certain someone's negligence." Nightmare cackles.

I simply stand to the side, not saying anything, staring at my two former friends. The goal of this is to make them think I'm somehow infected with Darkness, so we can bait them into trying to save me.

It's far too late for that, though. I've already made my choice.

And I'm going to rain blood on their little pity parade.

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(I'm going to update with a drawn design here after I finish making book covers for other people, and the design is described also by VictoriaLopez658!)