Dragon? Demon? Both?

Black Dragon Vs. Dragon Demon

(with Lucy, Gray and Happy, Natsu pov)
I was walking towards Lucy, when I felt the book flow into me, signifying the return of my Curse powers. "Yo" I walked up to my friends waving at them when Happy flew into my chest, literally. I patted Happy's back trying to calm him down, Gray gave me a questioning look and I nodded at him, surprisingly, he smiled warmly at me, I turned my attention to Lucy seeing her crying tears of joy and I smiled my trademark smile.
"Natsu~ thank goodness you're ok!" Lucy ran up to me and hugged me. I hugged back with the intent on calming her down "Did you think I would lose to my brother when he needed to be knocked down a peg? Of course I'm ok; at least, for now."
"WHAT'S THAT SUPPOSED TO MEAN!?" Lucy, Happy and Gray yelled at me in confusion. I honestly can't blame them, I'm still trying to wrap my head around the fact that I won't stay human for much longer.
"I'm not going to stay human for much longer. Remember the Dragon Seeds?" They nod in confirmation that they do remember. "Well, I had not only a Dragon seed, but also a Demon Seed. These two Seeds if left alone would merge and in effect kill me, however, instead of killing me, they only made me stronger when they finished merging. This is why I told you not to open my book, the book of E.N.D., because it would've resulted in the new seed sprouting and changing my physical form to that of a Dragon Demon" To say that Lucy, Gray and Happy were shocked would be putting it lightly, they just couldn't believe that I had done the impossible, again. Lucy had detached herself from me during the explanation.
Lucy was the first to recover, "So, you're going to become the most powerful being in earthland, is that what you're saying?"
"Putting it simply, yes."
"Even more powerful than Acnologia?"
"Probably."
"Damn it! After I get a major powerboost you go ahead and become more powerful than the only dragon currently in existence!" Gray exclaimed in frustration and a sliver of awe, though he wouldn't admit that out loud.
"Speaking of Acnologia and dragons, I can't smell the other dragon slayers, I can only assume Acnologia has taken them somehow" I put my hands out in front of me and summoned my book, the book of E.N.D., and opened it, to say that the subsequent pain was painful would be like saying a bee sting itches, it was pure torture, I could faintly hear my friends calling out to me, I told them to back off so that they don't get hurt. I then heard it, the roar of a dragon, his roar, Acnologia is coming, and the seed knew it, speeding up the transformation.
A few minutes later and the pain had stopped, I looked myself over and saw that I looked the same as I did when I was talking with Igneel. I looked at my friends and I heard Lucy say, "Natsu? Is that you?"
I looked directly into her eyes and saw golden ones looking back, I decided to give her the Dragneel smile "Of course it's me Lucy, who else could I be? END? That's impossible because he and I are the exact same person, no differences in personalities or morals." Lucy, Gray and Happy all relaxed at that.
"Now I get to beat the crap out of Acnologia, this'll be fun." I said with a shit eating smile. Lucy and Gray sweat dropped at my eagerness while Happy responded with an AYE SIR. I heard a roar in the distance that is unmistakable, Lucy, Gray and Happy had horrified looks on their faces "speak of the devil and he will appear!"
I flapped my wings a couple times, sending me into the sky, and headed towards the Black Dragon of the Apocalypse.
(Location: Hargeon, with Erza and the rest of the guild, Erza's pov)
I was shell-shocked when Acnologia destroyed all the boats in the port, he destroyed his weakness on pure instinct. Out of the corner of my eye I saw a small shape flying towards Acnologia, turning my attention to the shape, it looked like a dragon from this distance, but as it got closer it looked more and more human, and ever so slightly demonic, once it was close enough I saw a mop of pink hair.
Taking the logical route I determined that this being was in fact Natsu, and simply knowing it was Natsu gave me an immeasurable amount of hope. I pointed towards him "NATSU~!" everyone present, even Acnologia, looked over to where I was pointing and saw Natsu flying right at Acnologia, "FIRE DRAGON DEMON'S HORN!" Natsu engulfed himself in his normal dragon fire, but also this purple fire that seemed demonic, and crashed into Acnologia, who promptly roared in pain and annoyance.
(Natsu pov)
After slamming myself into Acnologia, said dragon turned to me and charged up his roar, deciding to not take the risk of responding with my own and losing I flew above him just before he fired. "FIRE DRAGON DEMON KING'S INCINERATING FIST!" Engulfing my fist with dragon demon fire, I struck Acnologia's back pushing him down a little.
Acnologia turned to me and responded with his own fist attack, I tried to dodge it but Acnologia was too fast and hit me in the chest. After stabilizing myself, Acnologia and I simply stared at each other, trying to intimidate each other, seeing neither of us were going to back off, I decided to use my curses, the whites of my eyes turned black and my irises turned red. Raising my hand so that the palm of my hand was facing Acnologia and a ball of orange fire appeared in front of my palm "SMOULDER" the ball of fire shot straight at Acnologia, who tried to eat the fire only to find out he couldn't, the ball of fire hit Acnologia square in the chest burning him slightly.
Acnologia sent another breath attack at me, dodging it I snapped my claw, a bunch of orange dots appeared around Acnologia, then exploded with orange fire, engulfing Acnologia, when the fire dispersed, Acnologia seemed to be missing a few scales. "ONIBANI: ARM!" A giant fist and forearm of orange fire formed around my right claw and forearm, I punched Acnologia with the arm of fire, taking off a few more scales.
Before Acnologia could do anything, I flew above him, forming a Leg out of the same fire as I had made the arm "ONIBANI: LEG!" I raised my leg up and kicked downwards, sending the black dragon to the ground, I landed next to him and noticed that his wings were bent at an awkward angle.
I breathed in, preparing a spell to finish off the Slayer of Souls, as I prepared my spell, Acnologia charged his breath spell to try to counter my own spell. The self-proclaimed 'Dragon King' launched his breath at me, just before the roar hit me, I unleashed my own spell "DRAGON SLAYER'S SECRET ART: HIDDEN FIRE FORM: ROAR OF FIERY SINS!" A torrent of golden flames left my mouth, destroying the 'Dragon King's' roar with ease and engulfing the black dragon of the apocalypse in the golden flames. When the flames died down, it showed a slowly disintegrating dragon body where Acnologia used to stand.