Igneel vs. Acnologia

Igneel vs. Acnologia is a fight fought between the Fire Dragon King Igneel and the Dragon King Acnologia.

Prologue
As the battle between Tartarus and Fairy Tail progresses, Erza finds herself against Kyouka, motivated to defeat her in order to stop Face. After they exchange several attacks, both them, as well as everyone else on the battlefield, take notice of sudden tremors. It is the Dragon Slayers that react to this strange phenomenon, with Wendy being the first one to fall down under unknown pressure. Elsewhere, Natsu hears a familiar voice, instantly connecting these happenings with the arrival of the Black Dragon, Acnologia. At unknown place, Igneel wonders if it is time to act yet.

As Acnologia descends, causing a great amount of destruction to the surroundings, the rest of the Dragon Slayers being acting strangely as well. It is not long before Natsu can hear Igneel's voice. As the Fire Dragon leaves his Dragon Slayer's body, apologizing for everything, he rushes to engage in battle with the Black Dragon, being watched by his teary-eyed foster son.

Battle
Both Dragons then proceed attacking each other, exchanging several blows, much to the awe of their spectators. However, Natsu sets his feet aflame to reach Igneel, demanding an immediate explanation. Telling him to be quiet, Igneel grabs Natsu out of his way and unleashes a huge amount of flames towards Acnologia, surrounding him with an explosion as the flames hit their target. Natsu is amazed to see such power, but Acnologia is unfazed. Igneel takes this moment to give Natsu "work", telling him to steal the book of E.N.D. from Mard Geer Tartarus, before the battle resumes and Igneel is seen hitting Acnologia with Fire Dragon's Iron Fist, with Natsu doing the same to Mard Geer.

Igneel then gets distracted for a moment, as Natsu gets overwhelmed by Mard Geer, barely managing to dodge Acnologia's attack. The latter expresses his anger that Dragons are still alive, promising to slay Igneel as his enemy. Afterwards, they attack each other several times, until Igneel begins ascending. Acnologia chases him, however, Igneel states that he isn't escaping, rather, now that they are above the clouds, he can go all out. Acnologia replies that this won't be a battle, but Igneel's destruction instead. However, as soon as Face begins its activation, slowly sucking up the continent's Magic, both Dragons fall down, with Acnologia seemingly being unconscious. Igneel encourages humans not to give up, just as other Dragons appear, destroying one Face after another. With the revival of E.N.D., as well as Face being stopped, he claims that victory is theirs.

Igneel then beings explaining the reason for Dragons being inside the Dragon Slayers, however, Acnologia gets up one more time and the two Dragons ascend to continue their battle. Acnologia thinks about Dragon Slayers being prevented from tranforming into Dragons and Igneel questions the Black Dragon's purpose, to which he replies that all he wants in a destruction. Slowly, but surely, Igneel begins getting overwhelmed and Acnologia takes notice of it, with the Fire Dragon noting that this is what happens when you sleep for that long. While continuing the fight, he contacts Natsu through Telepathy, letting him know that another reason for his disappearance was to defeat Acnologia. Natsu wants to assist him, however, Igneel refuses and charges at Acnologia, tearing off his arm. However, his own body takes a great deal of damage in the process and Natsu watches as his foster dad falls down, being hit by Acnologia's Dragon Roar afterwards.

Aftermath
Meanwhile, the other Dragon Slayers reunite with their foster parents. Sting and Rogue learn that their memory of killing the Dragons was manipulated and afterwards, the Dragons reveal that they are already dead, thus staying in the bodies of the Dragon Slayers being the only way to prolong their lives until now. At the same time, Natsu kneels down near Igneel's lifeless body, crying that he's now unable to tell him about all the things he wanted Igneel to know. The Dragons then part with humans, ending the era of Dragons. Just as Igneel is about to disappear as well, Natsu promises to get stronger in order to defeat Acnologia.