Natsu Dragneel & Gray Fullbuster vs. Mard Geer Tartaros

Natsu Dragneel & Gray Fullbuster vs. Mard Geer Tartaros is a fight fought between Fairy Tail Mages Natsu Dragneel and Gray Fullbuster, and the Underworld King of Tartaros: Mard Geer Tartaros.

Prologue
At the climax of his fight with Natsu and the Twin Dragons, after failing to get in contact with Kyôka regarding Face's activation, Mard Geer explains to Sting and the other Slayers that the reason he is trying to extinguish Magic Power from the continent is to revive Zeref's strongest Demon, E.N.D., which will allow them to return to Zeref's side. Mard continues his explanation by answering to a confused Rogue that the Etherious were all programmed with the instinct to kill Zeref and rid him of his immortality, taking the form of the command "return to [his] side". Mard finishes his explanation by stating that E.N.D. was sealed with Magic Power, and that as long as it exists, his seal cannot be undone, hence the need to activate Face; after they revive him, Zeref can finally die. Sting finds the fact that humans are getting caught up in Tartaros' objective, however Mard Geer dismisses his concerns, calling humans nothing more than insects. The Underworld King then creates a giant Prison Flower that, after being used for an initial attack, is frozen by an arriving Gray, who returns Natsu's lost scarf whilst also chiding him for losing something precious. Mard then notices that his left arm has been frozen and questions Gray's identity; Gray reveals the tattoo on his arm and identifies himself as the Devil Slayer who will defeat Mard Geer.

Those present quickly express confusion, 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 more Devil Slaying ice, which Mard nearly immediately recognizes as belonging to Silver. 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 Tartaros for his father, but before he can strike a critical blow the frozen Prison Flower bursts open and a demonic Jiemma stops Gray from killing the Underworld King. Over the top of everyone's shock, Mard Geer explains that Jiemma, his new "servant", 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 his sight; 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 Tartaros; and tells Natsu and Gray that he is Tartaros' "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.

Battle
Gray and Natsu then charge at Mard Geer, but in response to this, Mard spreads his wings and creates a considerably larger version of his Thorns that the two Fairy Tail Mages destroy. The Underworld King, however, uses this as a distraction to grab both Gray and Natsu's heads, fly into the remains of Cube, and grind their faces against the stone, all the while goading them. This, however, doesn't last long, as Gray and Natsu break free of Mard Geer's grasp and, hearing the pleas of the wounded Sting and Rogue, punch him into the stone they were just dragged along. Taken aback, Mard grabs Natsu and Gray and throws them to the ground, after which he kicks them both. The two Slayers then fire off their respective breath attacks, which Mard Geer easily dodges; he flies up and fires off the giant Dea Yggdrasil, which by combining their strength, Natsu and Gray manage to destroy.

Gray immediately freezes the chunks of Dea Yggdrasil and fires off his Ice-Make: Gungnir at Mard, which he also dodges, but quickly realizes conceals Natsu, whom blasts the Underworld King point-blank with Fire Dragon's Brilliant Flame, sending him rocketing towards the ground. However, to their annoyance, Mard Geer rises without a scratch, causing Natsu to question if he's invincible. Gray, however, tells Natsu that if his Magic can connect, they'll win; Mard ignores the two and tells them that they did well for providing him with amusement, as well as explaining that E.N.D. created Curses from The One Magic based on the negative emotions humans put forth: a power based on the very presence of life. Natsu tells Mard Geer that Magic is what will shape the future instead of Curses, but Mard Geer releases a strange mist that paralyzes Natsu and Gray, calling Curses the power that will triumph over Magic. He then reveals that the mist is part of the ultimate Curse he created to succeed in destroying Zeref: the memento of death; Memento Mori, citing that it does not kill, but simply erases the existences of those it touches completely, turning them into nothing. As the smoke begins to clear, Mard exits his Etherious Form and remarks that he perfected Memento Mori, and adds that once E.N.D. is revived, he'll finally be able to destroy Zeref.

Mard Geer's arrogance quickly turns into confusion, however, as within mere moments the Slayer duo is revealed to be otherwise unharmed, though Gray sports half of his body afflicted with strange black markings. This confusion turns to anger as Mard Geer realizes that Gray, using Silver's power, turned into a half-Demon being to block his Curse. Gray then collapses, entrusting the future to Natsu; Mard Geer, now completely livid, re-enters his Etherious Form and rushes to kill Natsu, though Natsu, also angered at Gray's sacrifice, spontaneously enters Dragon Force and hits Mard Geer away. The Underworld King questions Natsu's sudden boost in power, but after getting a good look at him and being hit with several more blows, he realizes that he is using the true power of a Dragon Slayer. In an attempt to completely decimate Mard Geer, Natsu uses Igneel's secret technique and Dragon Slayer Secret Art, Crimson Lotus: Phoenix Blade, which greatly injures and escalates Mard Geer's anger even further, prompting the wounded Demon to scream that he'll have to try better if he wants to defeat him.

Mard Geer then grabs Natsu's head and prepares to obliterate his body; in response, Natsu insults Mard Geer, but laments that his Magic Power has essentially reached the zero point. The Underworld King tells Natsu to be gone, however Natsu, ignoring him, mutters that he too has things he wants to entrust and shouts for a currently rising Gray Fullbuster, who molds a bow and arrow from ice and fires off his Ice Devil's Zeroth Destruction Bow, which pierces Mard Geer and leaves him mortally wounded due to the adverse effects Gray's Magic has on Demons. However, as Mard Geer falls from the sky, defeated, he converses with Kyôka, who tells her master not to fear, as Face's timer has just reached zero, signaling the end of the magical world as they know it.

Aftermath
Mard Geer hits the ground, unable to comprehend what has happened as Natsu and Gray congratulate each other. When Gray says he needs to destroy E.N.D.'s book, Natsu says he needs it for Igneel and requests that Gray stop joking. Gray, however, who has now picked up the book, claims not to be joking, citing that E.N.D. created Tartaros and that they should destroy Zeref's most powerful Demon while they can. The two have a brief stare-down after Natsu demands that Gray give him that book so he can give it to Igneel, however at that moment their personal problems cease as Face activates and begins sapping all Magic Power away from the continent. While this occurs, Mard Geer confidently states that what is happening is the beginning, and that when E.N.D. comes back to life, humans won't have the power to resist his will. With Sting and Rogue waking from their battle with Jiemma, Natsu screams in frustration; at that moment, Acnologia comes crashing down from the sky, wounded (leaving everyone bewildered), with Igneel soon pinning him down by the throat, requesting that humanity not give up quite yet.

Elsewhere, the Mages of Blue Pegasus attempt to destroy one of the Face bombs but fail miserably, their airship rapidly losing power by the second. It is then that someone telepathically tells them to move aside, which they narrowly do as that someone rushes through, and destroys, the Face bomb they failed to contend with rather easily. What they thought was a bird is in fact the Sky Dragon Grandeeney, who has also appeared alongside the Iron Dragon Metalicana and the Shadow and White Dragons Skiadrum and Weisslogia, the latter two of whom were thought to be dead. Igneel monologues their exploits as their foster children are all surprised at their reappearance, Mard Geer dumbfounded, and everyone relieved to see Face completely destroyed.

At this point, Acnologia begins to stir, unnoticed by Igneel, who says that E.N.D.'s revival has been halted and that victory is theirs, leading to a cheer of joy from Fairy Tail. Mard Geer, on the other hand, is still unable to wrap his head around Tartaros' defeat; Gray sees that E.N.D.'s tome has somehow made its way to the ground, some distance away from him. Sting then finally speaks, wondering how Weisslogia was alive, which Igneel reveals was due to all the Dragons hiding in their childrens' bodies using a secret art. When Natsu asks why they did this, Igneel says that it was to prevent Dragon Slayers from completely turning into Dragons like Acnologia, but before he can finish, Acnologia rises up and forces Igneel to continue their battle; as he departs, Igneel yells at Natsu to get E.N.D.'s book. Sting and Rogue wonder what he's talking about, and Gray picks up the book again and shows them; Natsu demands the book, but Gray reveals he also made a promise: a promise to defeat E.N.D., however the book is then swiped from his hand. The book thief is revealed to be Zeref, who claims that he is merely reclaiming his precious book.

Everyone marvels at Zeref's presence, but Zeref ignores them and instead addresses Mard Geer, whom he congratulates for being a step away from reviving E.N.D. Mard Geer's awe turns to something resembling sadness and fear when Zeref adds that it's time for him to sleep, where his silence breaks and he says that he only wishes to fulfill Zeref's desires. Zeref, however, puts Mard Geer back into his book, sets it on fire, which kills him, and claims that he cannot fulfill his desires, an act which leaves everyone mortified. Zeref responds to their outrage by saying that he no longer needs Mard Geer's help and then directs his attention to Natsu, where he mentions that he planned on finally settling things with him, but that Acnologia interfered with his plans. When Zeref questions whether Acnologia will once again end history or be stopped by a miracle, Natsu asks what he's talking about; Zeref ignores Natsu's question and instead tells him that should he make it through the hopeless situation he's in, then he shall sink him into even deeper hopelessness. Zeref then turns around and vanishes into thin hair, his departure leaving the four Slayer Mages angry.