Acnologia, also known as The Black Dragon of the Apocalypse, is an extraordinarily powerful dragon that roams the continent or world. It once clashed with Fairy Tail's strongest mage, Gildarts, and reportedly defeated him instantly, tearing out several organs in the process. Even Zeref, the most malevolent and potent mage globally, fears Acnologia due to its immense power.
There appears to be a profound connection between Acnologia and the Black Mage Zeref, as Zeref seems to know it well and sensed its approach long before it arrived. According to Zeref, Acnologia "Heralds the End of Eras," though the exact meaning remains unclear.
Acnologia arrives at Tenrou Island, causing chaos and battling the Fairy Tail members there. During the fight, it inflicts such devastation that Makarov must confront Acnologia in his Titan mode to allow other Fairy Tail mages to escape. As they battle, Acnologia eventually knocks Makarov down. Just as he is about to kill Makarov, all of Fairy Tail returns and starts attacking the dragon. Acnologia then ascends into the air and seemingly destroys Tenrou Island and all the Fairy Tail members on it with his "Dragon's Roar." After these events, Acnologia takes to the skies and vanishes from sight.
Acnologia was among the first Dragon Slayers created to participate in the Dragon Civil War. However, he and the other Dragon Slayers turned against them, slaying many dragons. Acnologia killed numerous dragons until he bathed in their blood and eventually transformed into a dragon himself.
While it is currently unknown which specific element Acnologia uses in his casting of Dragon Slayer Magic, this magic grants him the ability to reap the souls of dragons, leaving them in a half-dead state. From devouring and bathing in the blood of countless dragons, Acnologia can consume any type of magic element and/or "evil" aura to replenish himself.
(Source: Fairy Tail Wikia)
