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.

(Source: Fairy Tail Wikia)