Dragon? Demon? Both?

A Demon's Wrath

(in the real world, Natsu pov)
I sat up suddenly, surprising everyone in the room. "Natsu!" Lucy exclaimed, covering herself with the blanket. I looked around, then at himself, I was still my normal self, for now, then at Lucy. "Hey Lucy! Happy, how are you?"
Porylusica steps up and slaps me across the head "How are you alive! You should be dead!"
I rub the area where Porlyusica slapped me "I'll explain that later; right now, I've got to go kill Zeref." I stand up and got dressed; once I finished dressing myself I ran off towards Zeref. As I was running, I figured out that the seed hadn't sprouted, though, it still gave me a major power boost. Unless someone opens my book, I should be fine for now. As I approached the guild hall I could feel Gray using Iced Shell. I increased the speed at which I was running, hoping I got there in time.
(at the guildhall)
I burst through the doors ran up to Gray and tackled him "Stop!" I lit himself on fire along with Gray, warming Gray up who was practically a human popsicle at the moment, along with thawing out Zeref and the guildhall. Once I thawed Gray, I exclaimed, "don't tell me you've already forgotten! That spell...I've already stopped you from using it before."
"I-I let my emotions get the better of me, and I tried to kill you, my own guildmate. I don't deserve to be in the guild anymore! That's why...!"
"I'm not any different, you know! Tell me, Gray." I lifted my face, showing I was crying anime tears. "We're friends...aren't we?" Gray stared at me, speechless. I lowered My head again, hiding my eyes "don't die on me! Don't even think about dying! You have to live! Damn you..."
"But... if we beat Zeref..."
"You'll die" Zeref finished. "Whether you win or lose, you'll die. Fate is cruel, indeed."
I stomped my foot down in defiance "Not anymore. I've cut the magical ties that would kill me if you died. As long as that book isn't opened" I point at the book of E.N.D., my book, its cover had become scaly as if acknowledging me as a Dragon Demon "I won't have any major changes done to me." I turned to look at Gray, Lucy, who somehow got some clothes back on, and Happy "speaking of which, Lucy, take that book and get at least a mile away from the guild hall, and whatever you do, DON'T OPEN IT!"
"R-Right." Lucy picked up the book and left, along with Gray and Happy.
"Now then," I turned to face Zeref again "how about we finish this, brother."
Zeref smiles menacingly "Indeed we shall, E.N.D!" We stood motionless waiting for the other to make their move "my name is Natu!" Me being me, I launched myself at Zeref, spinning around and colliding my wrist with Zeref's, it became a battle of strength, I quickly overpowered Zeref and sent a barrage of punches at him, Zeref blocked them all.
I went for a roundhouse kick, barely getting a hit on my older brother, sending him across the guild. I quickly went after him, Zeref landed gracefully and stood straight, smirking. I paused a few feet away, waiting for my brother to make a move. Zeref appeared before me, I quickly feigned punching at Zeref, who just appeared behind me in response to the 'punch', and I spun around delivering a devastating haymaker to Zeref's gut.
Who was shocked that I anticipated him going behind him, at least, that is what I thought he was thinking. Zeref crashed against a wall, after landing on the floor debris fell on him. Zeref got back up appearing a little banged up "let me say that I'm shocked, I never thought you'd get the power to kill me after just resting up, what happened to you, Natsu?"
"I'm more than human, dragon, or demon. I'm a Dragon Demon." I ignited my body in not only my dragon fire, but with demon fire as well, which was purple in color. "With this power, I'll kill you, have a good rest, big brother" I charged at Zeref, pulling my fist back and concentrating the fire surrounding me into my fist "FIRE DRAGON DEMON KING'S INCINERATING FIST!" I punch Zeref in the gut again sending him through the guildhall's wall. Zeref stood back up with a few burns and his clothes being slightly singed, and a genuine smile.
"I can finally die, it seems too good to be true!" I rush up attacking with another 'FIRE DRAGON DEMON KING'S INCINERATING FIST' sending Zeref flying his coat burnt off and his top all but burned off.
"And to finish you off, FIRE DRAGON DEMON KING'S WRATHFUL ROAR!" A torrent of dragon and demon fire came from my mouth and flew at Zeref, when I was done, Zeref had second degree burns scattered throughout his body, Mavis chose this moment to appear.
"Natsu! What are you doing here!?" Mavis asked.
"First Master! Can I leave the rest to you?" I gestured at Zeref, Mavis looked at Zeref and nodded "then I'll take my leave" I started walking towards Lucy, Gray, and Happy. "I'm tired."