Acnologia

Acnologia, also known as The Black Dragon of the Apocalypse, is an extremely powerful dragon that flies around the continent or world. It once faced Fairy Tail's strongest Mage, Gildarts, in battle, and was said to have instantly defeated him, tearing out several organs in the process. Acnologia's power is such that even the most evil and powerful Mage in the world, Zeref, fears him. Acnologia seems to have a deep connection with the Black Mage Zeref, as Zeref seems to know it very well, and was able to sense its arrival quite a while before it actually arrived. It is said by Zeref that Acnologia 'Heralds the End of Eras', although what this means is still unclear. Tenrou Island arc Acnologia comes to Tenrou Island, wreaks havoc on it, and fights the Fairy Tail members on the island.[9] In the battle, it causes so much devastation that Makarov is forced to hold him off to allow the other Fairy Tail Mages to escape in his Titan mode.As the two fight, Acnologia is eventually able to knock Makarov to the ground. Just as he is about to kill Makarov, all of Fairy Tail comes back and starts attacking the dragon. Acnologia then flies into the air, and seemingly destroys Tenrou Island, and all of the Fairy Tail members on it with his "Dragon's Roar". After these events, Acnologia takes back up to the skies and disappears from sight. Acnologia was one of the first Dragon Slayers created to fight in the Dragon Civil War. However, he and the other Dragon Slayers turned on them and slayed them as well. Acnologia killed many dragons until he, with the help of Zeref, was turned into a dragon. It is currently unknown which type of Dragon Slayer element he formerly used.