Natsu Dragneel, Sting Eucliffe & Rogue Cheney vs. Mard Geer Tartaros

Natsu Dragneel, Sting Eucliffe & Rogue Cheney vs. Mard Geer Tartarus is a fight fought between Fairy Tail Mage Natsu Dragneel, Sabertooth Guild Master Sting Eucliffe and Sabertooth Mage Rogue Cheney, and the Underworld King of Tartarus: Mard Geer Tartarus.

Prologue
Following Igneel's subsequent emergence from Natsu's body and engagement in battle with the newly arrived Black Dragon of the Apocalypse Acnologia, Natsu, angry at Igneel having been inside his body all along and never telling him, rushes into the sky to interrogate his foster father, however he merely acts as a hindrance to Igneel, who angrily insists that Natsu silence himself before letting loose a massive Fire Dragon's Roar at the attacking Acnologia.

With Acnologia's next actions delayed, Igneel tells Natsu to leave; Natsu adamantly refuses on the grounds that he and Igneel haven't seen each other in over fourteen years. Igneel, though, merely repeats that they'll talk later and, remembering that Natsu is in a guild, directs Natsu towards Mard Geer and the book he's holding: the Book of E.N.D.; which he makes a formal request for Natsu to only steal, rather than open or destroy. Natsu questions why he should, to which Igneel responds that only Natsu can, and upon seeing his puzzled look, Igneel decides that he'll tell Natsu everything he wants to know if he can complete his request. Natsu accepts Igneel's request; Igneel throws Natsu towards Mard Geer, and during his fall he screams for Igneel to keep his promise, which Igneel accepts. Upon hearing these words yells that he's all fired up to the amused Mard Geer.

His smirk fading, Mard Geer asks Natsu who he is, and as he blocks Natsu's punch, the Dragon Slayer replies that he is Igneel's son, which surprises Mard. After hearing Natsu declare that he'll be stopping Face, stealing the book and defeating him as well, Mard attacks Natsu with his Thorns, but expresses shock when Natsu dodges them all and hits him square in the jaw with Fire Dragon's Iron Fist, just as Igneel does the same to Acnologia right above them. His awe turning into joy, Mard Geer then telepathically communicates with Kyouka and tells her to speed up Face's activation at the cost of her life, so that he may do away with the Mages opposing them, as well as sap away the Dragons' life force and revive E.N.D.

After the fact, Natsu calls out to Igneel and charges at Mard Geer again, only for Mard Geer to insult him and fire off an explosion at him, citing that only the Dragons are obstacles to him. Natsu manages to dodge the explosion and tries to kick Mard Geer with his Fire Dragon's Claw, which Mard Geer blocks before subsequently smacking Natsu underground whilst claiming that he is certainly no Dragon. While Igneel and Acnologia's battle above them escalates, Natsu is nearly killed by Mard Geer's Thorns but is saved by the timely arrival of Sting and Rogue, who shatter the spell. After a mild amount of banter between them, Sting proposes to Natsu that they join forces to defeat Mard Geer, but he refuses, stating that he wants to complete Igneel's request alone. However, Sting manages to trick Natsu into cooperating with them by turning the fight into a competition to see who can beat him first; Natsu rushes toward Mard Geer as Sting and Rogue mention how irksome it was to convince him to accept their help.

Battle
Natsu begins by throwing a flaming punch at Mard Geer, but he is dodged and countered with a hand chop to the head; Sting makes a similar elemental jab, but he too is knocked away. Rogue slashes at Mard and Natsu uses Mard's focus on dodging Rogue's attack to kick him in the face. Rogue then follows up with another attack and lands a successful hit on the Underworld King, as does Sting, who jumps in front of Mard and kicks him through a wall. As the Dragon Slayers stand united, Mard Geer says that before him are the children of Dragons while two real ones clash in the sky; adding that there may yet be more that will fly about his precious Cube as he pleases, Mard Geer says that he has recalled what it means to be angry, which Natsu uses to insult the Tartarus member, telling him that he's now on the same level as humans.

Mard proceeds to tell Natsu that emotions hinder thought and that he got rid of his for such a reason many years ago, but adds that they made him snap. Mard then sets E.N.D.'s book down and apologizes for having to make him wait longer, as well as explains that anger causes one to deviate from their true personality, and that the thought of him no longer being himself is appalling. Stating aloud that he is the Underworld King and caretaker of the masterless Tartarus, Mard Geer tells the Slayers that when he kills them, he will also once again kill his emotions. With the Dragon Slayers sensing what he's about to do, Mard releases a large number of his Thorns that swirl around and slice the Dragon Slayers. Frustrated, Natsu grabs one of the Thorns and uses it to pull himself closer to Mard Geer, but as he is about to attack he is blown away with a wave of Mard's hand; Sting and Rogue are then attacked by an explosive rose flower. Angered, Natsu hits Mard with his Fire Dragon's Sword Horn, but Mard feels nothing and tells the Slayer that his attack is ineffective, however Natsu increases its strength by adding the momentum of a Fire Dragon's Roar, which launches Mard high into the air; he then requests Sting and Rogue help him.

Below them, Sting and Rogue, per Natsu's plea for help, guess that Natsu must have finally realized that Mard Geer cannot be beaten alone and launch their Unison Raid: Holy Shadow Dragon's Flash Fang; an attack which Mard grabs the brunt of with his bare hands and completely negates while calling it beautiful. From above Mard Geer, Natsu uses his diverted attention yet again to land his Lightning Flame Dragon's Firing Hammer, but rather than be injured or sent flying, Mard remains stationary and smirks at Natsu before hitting him away with his Thorn Curse. His confidence still brimming, Mard lands on the ground and sits in his throne, sending an arrogant gaze towards the Dragon Slayers as he does so, as well as mentioning that they should give up, as he has yet to even use his Etherious Form.

From there, Mard telepathically contacts Kyouka again to ask her how Face's activation is coming along but receives no reply and says that she must've forgotten his voice in her fun, but adds that she is doing good work regardless. Sting then goes on to ask Mard Geer why he's trying to erase Magic Power from the continent, which he is told is to revive E.N.D.: Zeref's strongest Demon; which will allow them to return to Zeref's side. Rogue, confused, tells them that they can just go by themselves, but Mard explains that the Etherious that Zeref created were all programmed with the instinct to "return to his side", which means that they, his creations, need to kill him per Zeref's desire to die and rid himself of his eternal youth and immortality; Mard also says that the Etherious were created for just such a purpose.

Mard continues on, explaining that E.N.D. was Zeref's ultimate masterpiece, but that he was somehow sealed away and that in the Magic Power-filled Earth Land, E.N.D. cannot be revived as his seal is composed of that same Magic Power. After Mard cites that with Face's activation, E.N.D. will be revived and Zeref will finally die, Sting says that he's getting humans caught up in his mess, but Mard dismisses his concern, calling humans mere insects in his path. The Underworld King then creates a giant Prison Flower that knocks Sting, Rogue and Natsu away, but before it can do any more harm, it becomes frozen. With everyone awing at the ice, Natsu's missing scarf suddenly appears around his neck just as Gray reveals himself and chastises Natsu for losing Igneel's precious gift. Mard Geer notices that his left arm has been frozen right after this and, as such, questions who Gray his; Gray reveals the tattoo on his right arm and reveals himself to Mard Geer as the Devil Slayer who will defeat him.

Aftermath
At this revelation, everyone becomes confused, with Rogue drawing the conclusion that Gray must have attained a Magic that will allow him to truly defeat Mard Geer. Gray then strikes Mard with a large amount of Devil Slaying ice, which Mard immediately recognizes as having been Silver's Magic. Mard breaks out of the ice and mentions that he knew that Silver would betray them eventually and attacks Gray with his Thorns, however Gray freezes these as well. Gray brushes off Mard Geer and hits him with more ice, stating that he will annihilate all of Tartarus for his father before trying to deal the finishing blow, but at that moment, the frozen Prison Flower breaks open and a demonized Jiemma interrupts Gray's attempt at killing the Underwrold King. Over the top of everyone's shock, Mard Geer explains that Jiemma chose to become a Demon, unlike his daughter Minerva.

Jiemma, angry at Mard Geer for referring to him as a servant, rushes at Sting and Rogue, who express fear at him; Natsu tries to rally them to fight back by attacking Jiemma, but he is easily thrown aside. Sting and Rogue are attacked by their old Guild Master but cast their fear aside and fight back. As this occurs, Mard laments that Jiemma cannot yet readily control his emotions and tells Gray, who claims Mard Geer as his prey, that he cannot boast quite yet. As he transforms, Mard asks Gray if he thinks his Magic will continue to work against his true form, and says that like E.N.D. is about to, he too should awaken. Entering his Etherious Form, Mard reveals his true, full name: Mard Geer Tartarus; and tells Natsu and Gray that he is Tartarus' "Definitive Demon". In response, Natsu and Gray claim that they'll defeat him in the name of Fairy Tail; Mard Geer tells the two to come forward, as theirs shall be the final battle.