No POV

(Y,N) started to open his eyes and he was in black void was all around him he try to move his body but it wouldn't move. He was thrashing around but his body still wouldn't move so realizing that there was nothing he could do he just sat there in the dark abyss.

His mind started to wonder any keep wandering until he felt a intense pain inflicted upon his head.

It was a burning sensation and shortly after that his body felt like he was being stabbed with millions of tiny needles injecting Moulton lava into his very veins.

He try to scream out in pain but his voice was silenced in this Dark Void he could do nothing more than sit there and bear and grin the pain.

Slowly he felt his body started to change he feel his body become denser but at the same time stronger.

His head felt like it was going to split in half eventually everything went black for me again is the cold dark Void took me away.

???: (Y,N), (Y,N), (Y,N)

He started to open his eyes to look up at a stone ceiling slowly he started to sit up. He looked over to see serana with the same old smile on her face.

He grabbed his head and was groaning in pain like he felt like a Jackhammer just pummeling into my brain.

(Y,N): w-hat the f*** just happened?

serana: look for yourself.

She extended her hand out towords what looked at to be clear water slowly making his way over there being mindful of each step he took.

He look down into the water and was shocked at what his reflection was.

( not my picture's and this is you now with your wings)

(Y,N): the hell happened to me

Serana: you've been Reborn as a demon

(Y,N): so what do we do now

Serana: we wait for your friends once they awaken from their transformation we will proceed to go over the training.

And so to her words he and Serana waited for his three friends to wake up.

The next one to wake up was natsu as he stood out of the water they could get a clear look at him.

(Not my picture and that's him with his wings)

Natsu: god dammit ow my head's killing me.

(Y,N): Natsu

Realizing that voice although a little bit different natsu look over to see (Y,N) but with a completely different look.

Natsu: (Y,N) is that you

(Y,N): Ya and before you start saying I look different you should look at you.

(Y,N) signaled Natsu to come over to him. So natsu walk over to (Y,N) to see his new look due to the waters reflection.

Natsu eyes widen clearly in shock and his new look.

Natsu: thats me....AWESOME I LOOK SOO COOL

(Y,N) and Serana just giggled/ laughed at his behavior.

Serana then gave Natsu the quick rundown of what everything that's happened. He was sad not to be human anymore but if it meant getting his revenge he was more than happy for it.

So the three of them waiting for the next person to wake up and thats was gajeel.

Rising out of the liquid just like Natsu did we got a look at his New appearance.

(This is gajeel new look and none of these are mine)

Gajeel: oh my God where the hell am I.

Natsu: Hey Metal face

Gajeel turn to see the three of us standing there with our new look and he clearly had questions.

Gajeel: who are you two.

(Y,N): God you're so stupid sometimes it's me (Y,N) and this * extends hand towards natsu* and that's natsu.

Gajeel: the hell happened to the three of us!!!

(Y,N): I can't really explain Serana could you please explain to him and please use small words his brains to Tiny the process anything large.

Gajeel: Fuck you shadow lizard

(Y,N): yeah yeah good to see you too.

Serana just giggle before explaining to gajeel what's going on and what happened.

Once gajeel understood everything he walked over to the for them waiting for wendy.

It took about five minutes before they heard movement in the black liquid they turn to Wendy and her new look. And out of all of them she definitely change the most.

( not my pictures but she just slightly taller than her original hight)

Wendy: where am I (Y,N) natsu gajeel were are you?

Gajeel: over hear squirt

Wendy looked over to see the other three with their new looks and was confused.

Wendy: (Y,N), Natsu, gajeel is that you what happened to you three.

(Y,N): * Chuckles slightly* before you start comparing our old differences you should look at yourself.

So just like the other three she walked over to see her reflection. And she was quite structure new appearance and happy at the same time.

Why are very important reasons that I will not go into but lets just say that she finally "full out".

Though she barely had enough strength to walk over when she saw her reflection her leg started to wobble and she almost collapsed. If it wasn't for (Y,N) catching her.

Serana: well it seems the four of you survive your transformation. Now if I were you I would take the next day to relax. Your body's had just undergone and intense transformation. And if you try to overexert yourself you will most definitely die.

the four dragon slayers nod their head. So the five of them left the cave with (Y,N) giving Wendy a piggyback ride.

Once they were back in their separate rooms as soon as they hit their bed they were out.

They would only wake up two days later once their body fully recovered and they were well rested.

When that time came Serana sent hellhounds to come get them. But now that they were demon's they could understand them.

So the four dragon slayers all gathered out side the Castle.

( this is not my picture and ignore the person)

Once everyone arrived now Serana turn to face them.

Serana: I'm glad you all could join now for your training schedule we will be breaking this down into four parts. Part one will be focus on demon culture and also increasing your magic Reserves the magic I will be teaching you will require a lot of magic. Then part two is about building your physical body because the stronger your body is the more how are you can draw out. Part three will be learning your new Magic. And part four which will be the hardest is will be the training you will have to switch between human and full demon

(Y,N): human and full demon?

Serana: yes you would need a human body in order to sneak around the human world when we Break Free. But your demon form will be the Form where your powers are at the highest it can possibly be.

(Y,N): I get it so then what are we waiting for let's get this going so we can take our revenge.

Serana: that's good also just to tell you time works differently in this realm. In your old world One year is equivalent to 10 years in this world.

(Y,N): So we have a lot of time on our hands.

Serana: correct now let's stop wasting time and get on with it.

Slayers: yes ma'am

Four years later ( AKA 40 years in this world) no POV (sorry but I'm not putting in the training it's training who wants to read that)

It has been 40 years in this realm which is 4 years back in the human world and the four dragon slayers have grown tremendously more powerful.

They were able to get the rundown on demon culture some faster than others (*cough*Natsu*cough*) but they progressed nonetheless they're magic reserved we're 5 times that of a dragon King's so it breaks down to. They have a f****** ridiculous amount of Magic they can use.

They also grew a lot stronger physically Wendy even though she's not the physically strongest. Can still lift 30 times her own weight natsu and (Y,N) being more physical when it comes to fighting can lift about 5 tons easily. With a basic strike they can shatter large Boulders ten times their size.

They even started to move into their new Magic. Each of them were being taught two forms of magic God Slayer and demon slayer magic.

Wendy is learning Sky God Slayer and sky demon Slayer magic with some other forms of magic like better healing Magic any more high-level enchantments. And a new magic she is creating herself called storm magic.

Natsu started to learn fire god Slayer Magic and fire demon Slayer Magic. He even started to master multiple different forms of Fire magic and creating some of his own fire magic as well.

Gajeel what is learning metal god Slayer and metal Demon Slayer magic. He started to learn different forms of Earth Magic and was becoming pretty powerful with Earth Magic.

Now (Y,N) he had mastered the basics of Darkness God Slayer and darkness Demon Slayer. He even learn Shadow Magic. He starting to learn teleportation and requip. He even started to mix his Slayer magic together to create more powerful forms of them. He was slowly  growing to be the most powerful out of the slayers.

Their personalities also change to blend of demonic/human they were cold ruthless and unforgiving in battle and if they didn't know you or trust you they flat-out ignore you. God forbid you get one of them mad they will legit torture you be a mental trauma or physical trauma.

They weren't the same old Dragon Slayers anymore these were the dragon slayers turn demons and they were fueled by vengeance.

But once (Y,N) got to a certain point he could feel his body becoming stronger then that's when he first entered his full demon form.

Flashback one month earlier

The Dragon Slayers for doing their normal routine schedule improving their magic getting stronger reading themselves for Revenge.

Then something started to happen to (Y,N) he start to get surrounded by his Darkness. As it completely consumed him the other Dragon Slayers looked on in horror as they thought he just died.

But with a violent burst of wind the darkness went everywhere around them as they try to Shield themselves from it but being pushed back.

When the darkness settled there was (Y,N) in his new form.

( these are not my pictures in the wings are black red and silver)

Serana who is now just reached the scene was now able to look at (Y,N) in his new form.

(Y,N): What the hell happened?

His voice was not that of a human's anymore it was demonic dark the very voiced itself screamed authorities and power.

Serana: well it appears you have unlocked your demon form congratulations (Y,N) for being the first one to unlock it.

The first dragon slayer to snap out of his shock was natsu has he ran up to (Y,N).

Natsu: Bro you look so cool wait how strong are you in this form.

(Y,N): let's find out I'm going to try out a new technique I've been working on.

So with that the Slayers took a step back as he started to charge up his Roar. But it wasn't a normal Roar he was combining dragon slayer Demon Slayer and God Slayer all into this one attack combined with his demon form.

The four of them can't even possibly comprehend how much damage he was about to cause with that amount of power. But he was going to shocked everyone.

(Y,N): King's Demon God Dragon's: secret art: Annihilation Roar!!

He then fired his Roar at a nearby mountain range.

( make the beam black and red and this is not my gif)

He and including the others where in shocked at the destruction he had just cause what a single attack!!!

(Y,N) reverted out of his demon form and collapsing onto his knees panting heavily. The three dragon slayers rushed to him and help him to his feet.

Wendy: are you okay (Y,N)?

(Y,N): yeah I'm alright just that formed took a lot out of me not to mention. That attack I just use took even more out of me as well. I'll be fine I just need some rest.

Serana: yes I do believe you earn some rest (Y,N) you may go.

And with that he left Serana then turn to the rest of them and they knew that they need to get back to training.

Flashback end

After that the three other dragon slayers were able to enter their demon form and would usually Spar in them.

And that's is what they were doing right now.

(Y,N):

(Not my pictures)

Natsu:

( make Natsu in his demon form have pink hair and remove the Hood)

Gajeel:

(Not my picture and remove the necklace)

( not my pictures)

The four of Them would fight each other and their full demon form or demon king forms as Serana call them. By fighting each other and then get a better control over them. They would go for hours if not days fighting each other in these forms.

But they knew that they were coming close to being completed with their training maybe give them three years (30 years in this realm) and they should be done with their training.

Fast forward 2 years (AKA 20 years)

It was the same old but something strange happening lately it was (Y,N) his Powers have been looking like they're trying to evolve again. Like the first time he entered it Demon King form.

You can tell if your powers are trying to evolve due to the way it works. Their magic is a unique brand of magic because the more powerful the user is their magic adaps with them. Just like with dragon slayers is normal until a Slayer kills a dragon allowing him to go into Dragon Force. Certain criteria that have to be met for the evolution of something to happen. But (Y,N) magic look like it was trying to reach a level Beyond that.

Serana did some investigating and she was quite shocked at what was going on. When she found out what was going on she smirked oh how exciting this was for her.

Back with the Slayers they were training with natsu vs (Y,N) and gajeel vs wendy.

As they fought each other the four of them had the big as a Smiles on their face going up against each other they could go all out.

However the fighting stopped once Serana called for (Y,N)

Serana: (Y,N)!!!

(Y,N): what is it Serana?

Serana: I know what's going on with you and I think you might be quite shocked at the news.

(Y,N): cool then what's going on with me and how do I fix it?

Serana: that's simple enough to answer how you fix it is simple you must go Beyond the form you are now and you must.

(Y,N): must what?

The other dragon slayer stop their fight wondering what was going on as well.

Serana: transcend you're being.

There you go that's another chapter over.

And yes I left you in a cliffhanger because I'm starting to become evil this is what evil authors do alright bye bye.