A Black dragon, fearsomely reputed as the Dragon of the Apocalypse, that travels around continents and even the world. It appeared on Tenrou Island to, according to Zeref, bring about the end of an era.
Acnologia is first mentioned by Gildartz as the "black dragon" that he had encountered and fought on Zonia Mountain during his 100 year mission. However, it was so powerful he easily lost to it, and consequently lost a limb and an organ to the beast.
The monster then proceeded to appear on Tenrou Island after Zeref utterly destroyed Hades. Acnologia tore apart the island and Fairy Tail as well. Master Makarov attempted to give his life to save his guild, but they all run back like the family they are to save Makarov. After being hit by all three Dragonslayers, Acnologia attempted to annihilate the island, and flew off after it appeared to be vaporized. However, as Acnologia most likely doesn't know yet, Mavis was able to cast Fairy Sphere, protecting them from Acnologia's deadly attack.
In one of the recent chapters, a dragon named Zirconis revealed that Acnologia used to be a Dragon Slayer Mage that bathed in dragon's blood. Acnologia was transformed into a dragon by Zeref, and it is unknown whether this was as an ironic curse or a cruel blessing. This is possibly the reason why the Dragon slayer magic did not affect Acnologia, since it was not a real Dragon.