E.N.D.

"Perhaps you can surpass me... try... Natsu. Or should I say... Etherious Natsu Dragneel."

- Zeref Dragneel revealing the true name of E.N.D.

"Animus: Now I see. I know who you truly are... Neither Human nor Dragon... You're the Destroyer of All, E...N... Natsu: Who cares?! It doesn't matter what you call me... Because I'll throw it all away... If it means that I can save my friends! (Animus focuses the energy from a series of Magic Circles onto himself) Natsu: If I didn't have them by my side... My life... WOULDN'T MEAN ANYTHING!!!"

- Animus identifying Natsu Dragneel as E.N.D. only to be told off by him regardless.

Etherious Natsu Dragneel (エーテリアス・ナツ・ドラグニル, Ēteriasu Natsu Doraguniru), simply known as E.N.D. (イーエヌディ, Ī Enu Dī) was the original identity of Natsu Dragneel (ナツ・ドラグニル, Natsu Doraguniru), one of the main protagonists of the  manga/anime series who was reborn as a demonic Etherious thanks to his older brother Zeref Dragneel.

Originally Natsu who died about 400 years ago, he was later revived as Zeref's greatest Demon thought to be strong and powerful enough to make even Acnologia fear him and kill his own creator.

Personality
According to the Dragon Atlas Flame, E.N.D. was said to be the most vile out of Zeref's creations; as such it was safe to assume that while his Human form of Natsu was noble and kind yet fierce and cunning E.N.D was considerably much more ruthless and cunning, though to the point of having a deadly bloodlust.

His capacity for evil was enough that it was said to strike fear into the heart of the likes of Acnologia himself. After being awakened due to Natsu's bonds and Humanity, E.N.D. was apparently influenced considerably as he still desired to protect his friends above all else but retained a great amount of violent tendencies as he brutalized Dimaria Yesta in a one-sided fight to save Lucy Heartfilia.

While keeping his desire to destroy his brother and creator, this was partially influenced due to Lucy seemingly being killed. E.N.D was also more than capable of resorting to violence even towards his friends as he attacked Gray Fullbuster without a second thought or moment's hesitation for being in his way.

Appearance
E.N.D.'s appearance was the same as Natsu's, albeit with monstrously demonic traits such as a long reptilian arm and leg, a draconic wing and a long horn sticking out of his head on the left side of his body; at least half his face was covered in dark red scales resembling those of a Dragon. Upon undergoing his full Transformation he gained another horn on the other side of his head, his eyes turned yellow with black slits, even more of his face was covered in scales and the Fairy Tail emblem on his right shoulder was replaced by the Tartaros emblem.

Backstory
400 years ago, Natsu was born as the younger brother of Zeref but died at a young age alongside their parents courtesy of a Dragon attack. This caused Zeref to enroll at the Mildian Academy of Magic and research Magic Power and its relations to life, death and the Human race. Much later, with Zeref using his dead body he was revived as the strongest Demon he had ever created: E.N.D, whose purpose was to kill a disheartened Zeref who had become a Cursed and immortal Black Wizard in his quest for a second chance at life through Magic. As E.N.D., Natsu was a Demon specializing in Fire and Flame Magic and purportedly discovered a different route that Magic Power could take when it was still developing from the One Primordial Magic and Abyss of Sorcery, thus creating Curse Power: a Power entirely different and independent from Magic fueled by the darkest of emotions.

During this time he also met the mighty Igneel while with his older brother but the Dragon chose not to kill him because he loved him too much to do so; Igneel preferred to say that he could not destroy E.N.D. because of the Demon's true strength, though it is possible this false claim could've been verified if he really chose to fight E.N.D. However, E.N.D. was prevented from properly manifesting any further at some point by a Magic Seal that could not be broken through any means available around the time.

From that point on Igneel taught Natsu how to talk, write and use his own signature form of Lost Magic called Fire Dragon Slayer Magic; Igneel ultimately chose, like four other Dragons of that time period to Seal his damaged Soul inside Natsu's body and leap 400 years into the future, recover his strength with the high Ethernano concentrations and kill Acnologia. While initially thought to have been abandoned by Igneel on July 7, X777, it was actually the day and year he awoke in the future like the other Dragon Slayers.

The departure of E.N.D. from the past also gave rise to many legends about the strongest Demon from Zeref's use of Magic including his creation and leading of Tartaros (when Mard Geer Tartaros happened upon the Book of E.N.D., using it to unify the Etherious and other Demons under a single cause and created the Dark Guild, possibly naming it after himself) and the fact he was the most vile creation in the Books of Zeref, the Black Wizard's library of works.

Tartaros arc
Believing E.N.D. to be in a dormant state, Tartaros planned to use a weapon called White Inheritance: Face (a Magic Elimination System consisting of a series of Anti-Magic Pulse Bombs originally created by the Magic Council as a deterrent against the forces of Zeref's Alakitasian Alvarez Empire) to wipe out all Magic Power on the continent of Ishgar so they could gather enough Curse Power to revive their Master and "return" to Zeref with him.

Mard Geer was seen on his throne holding E.N.D.'s Tome as he talked about Zeref's Demons the Etherious: beings superior to Humans. Seemingly reassuring the Dark Guild's Master, Mard Geer stated that the darkness of the "Netherworld gate" would soon open and eat up the world's light.

Following the lethal deactivation of (only one) Face by Wendy Marvell and Carla, Kyôka reported to Mard Geer urging him to consider reviving their Master through more forceful means to assist them. However, the Underworld King informed her there was too much Magic Power present in the world for E.N.D. to properly resuscitate and though Kyôka reminded him that was the entire point of using Face, he stated their plans would still come to fruition.

Mard Geer would later go on to explain to Natsu, Sting Eucliffe and Rogue Cheney that E.N.D. was actually Tartaros and the Etherious' real Master, Natsu wanted to take the Book supposedly containing him and it was Magic Power itself that kept him Sealed away causing the Fire Dragon Slayer to realize the Etherious' goal of using the remaining Faces to erase E.N.D.'s only means of imprisonment and unleash him on the Magic World once more so they could meet their own maker; the Underworld King also told Natsu and Gray the story of how E.N.D. discovered Curses in the earliest days of Magic when it was still undeveloped.

Zeref reappeared after Mard Geer's defeat and the Face System's destruction to interrupt the argument between Natsu and Gray on who would take the Book of E.N.D. He summoned the Book to his hand, claiming it was something very important that belonged to him. Zeref then praised Mard Geer for his attempt to revive E.N.D and when Mard Geer stated he could realize what Zeref wished for, the Black Wizard turned the fallen Demon into another Book and burned it. He then vanished yet again, taking the Book of E.N.D. and seemingly the Demon with him as well.

Later at an unknown location, Zeref ominously remarked that Natsu had to try and overtake him, though not before calling the Dragon Slayer "Etherious Natsu Dragneel" revealing that Natsu was actually the Demon itself.

Fairy Tail: Dragon Cry
During the final battle with Animus, Natsu unwittingly unleashed the Power of E.N.D. for the first time in over 400 years after being mortally wounded by the Dragon with the Power of the Dragon Cry. Despite his seemingly-incomplete form Animus soon recognized Natsu as E.N.D., proclaiming him to be a harbinger of doom (though the Dragon was cut off before he could finish) and the Dragon Slayer used his new Powers to defeat Animus. Natsu then returned to normal soon after the fight, though he was left utterly shell-shocked by his Transformation.

Alvarez Empire arc
During a fight between Zeref and Natsu the latter revealed his relationship to Natsu and the Dragon Slayer's status as E.N.D. Though Natsu refused to believe it at first he was forced to after Zeref pierced a hole through the Book of E.N.D. and Natsu ended up hurt as a result which proved the elder Dragneel's story. After Zeref revealed what really occurred in the past Natsu lost his patience and attempted to kill his brother as he was told that if Zeref died it would possibly mean his own death as well. Natsu ignored this and declared his intentions of wanting to put a permanent end to Zeref only for Happy to fly away with him.

Natsu and Lucy were then captured by Dimaria, one of the Spriggan 12 who spent her time torturing Lucy as Natsu remained unconscious. Just when Lucy was about to have her eyes stabbed out the Power of E.N.D. was then unleashed inside Natsu and allowed him to break free of his restraints, completely able to move while in range of Dimaria's Time Magic and proceeded to brutally beat her saving Lucy. After defeating Dimaria, Natsu/E.N.D. went to check on her but after failing to wake up the unresponsive Lucy (who was still affected by Dimaria's Magic) he broke down which caused his instincts and Powers to possess him. He escaped the chamber and started hunting Zeref as he pinpointed him and began to make his move only to be confronted by Gray. Gray and E.N.D. mercilessly attacked each other without any hesitation. The Ice Demon Slayer and Fire Demon seemed evenly matched until they were stopped by Erza Scarlet. Erza made the two come to their senses and Natsu's Transformation diminished.

Some time later Mavis Vermillion obtained the Book of E.N.D. She then met up with Lucy, Happy, and Gray and lent them the book. Happy pointed out it was Natsu's book and Lucy realized she was literally holding Natsu's life in her hands. Mavis then leaves them to work on a plan to beat Zeref. Lucy, Happy, and Gray then sit on a bench looking over the book. They soon decide to open the book and when they do Letter Magic flows out of it. Lucy was then shocked about what it said.

Lucy tried to rewrite the words in the Book but became affected by its Black Magic. Gray then helps her get through the pain and continues to rewrite the book. The book's changes affected Natsu during his fight with his brother; they gave Natsu the edge he needed to defeat Zeref.

Later, Zeref died peacefully with Mavis and due to the Book's link with Zeref it disappeared. When the Book disappeared Lucy claimed that she did all she could to help Natsu and worried that he could disappear along with the Book. However, Natsu showed up where Lucy and the others were saying the fight was over.

Around that moment Acnologia managed to escape the Time Lapse and with its Magic Power he pulled all remaining Dragon Slayers into the Ravines of Time to face the former Demon and his comrades. However, this caused Gray, Happy and Lucy to believe Natsu disappeared because of his link with his now-deceased brother. This caused Lucy to drop to her knees crying believing that Natsu was gone forever, not knowing he was still alive and well while battling Acnologia's Human form while the Black Dragon's Dragon form came out from the Ravines along with a weakened Ichiya Vandalay Kotobuki and Anna Heartfilia instead of his Human form despite linking their minds and lives with the latter form coming to destroy Magnolia. As Lucy found the Fairy Sphere Magic guide book Mavis left behind Meredy used her Maguilty Sense Magic to connect every one of Fairy Tail's allies and former enemies both near and far; the Fairy Tail Guild and their allies used their bonds to strengthen the Magic Seal on Acnologia's Dragon form, also affecting his Human form and allowing Natsu to finish Acnologia once and for all; the dying Black Dragon deemed Natsu worthy of inheriting the title of Dragon King but the son of Igneel happily denied this for his peaceful desire to be with his friends. As the war officially ended Natsu and the other Dragon Slayers returned to the real world safely as Acnologia's Human and Dragon forms disappeared.

Powers and Abilities
E.N.D. was stated to be Zeref's greatest creation and the only one actually capable of slaying the Black Wizard. Silver Fullbuster later revealed to his son Gray that E.N.D. was a Fire Demon. E.N.D's Powers were so great he surpassed some of the Gods in the series as he was able to easily and freely move about when Dimaria, a Spriggan Shield stopped Time using the Power of the God Chronos and easily defeated the extremely-powerful Wizard who was on the same level as former Emperor of Ishgar-ranked Wizard Saint and Natsu's fellow Dragon Slayer God Serena, his might leaving her in absolute awe as she wasn't able to believe he could defy the Power of Gods. He was said to be so powerful that even the Black Dragon Acnologia had a fear of the Fire Demon. However, it appeared that due to being Sealed by the Magic Power in Earth Land he could only manifest for a while and reverted to Natsu's maximum Power after that. When fully charged E.N.D. was capable of taking on (and bringing down) the Dragon Animus, who was having his own Power amplified by the artifact known as the Dragon Cry at the time.

As Natsu, E.N.D. could use Fire Dragon Slayer Magic, a type of Lost Magic and form of Dragon Slayer Magic which allowed him to manipulate fire as well as be immune to and even consume it to regain energy; unfortunately, he couldn't eat or resist his own Magic Flames or the kind produced by a Flame God Slayer (unless under special circumstances) and the consumption of any slightly non-Flame Magic normally made him sick to varying degrees after he first used and depleted the Power it gave him; the overall physique his abilities granted him also made him prone to motion sickness, a side-effect of the biological and physiological incompatibilities of Human and Dragon senses.

E.N.D. could also (theoretically) use Curses as Mard Geer stated that E.N.D. was the one who discovered and created Curse Power. However, E.N.D. hadn't been shown using Curses so this is up for debate.

Trivia

 * Natsu's guise as the "Demon Lord Dragneel" in the Edolas arc's finale ironically foreshadowed his origin as E.N.D.; also, Natsu was originally meant to have horns, perhaps as a nod to his true (half-)Demon nature.
 * Before and after this it was noted by many of his opponents that out of all the Fairy Tail Wizards, Natsu was the one who always seemed the most like a vicious monster when fighting due to his savage ferocity and overall lack of restraint.