Acnologia

Acnologia (アクノロギア Akunorogia), also known as The Black Dragon (黒龍, Kokuryū), and fearsomely reputed as The Black Dragon of Apocalypse, is a powerful dragon that appears to be flying around the continent or the world. He appears to have some connection with Zeref; the depths of this relationship are unknown.

Appearance
Acnologia is a mastodonic dragon whose entire upper body is covered in dark, round scales, in turn decorated by spiralling, light-coloured markings all over them. Its lower body, specifically its belly, inner tail and legs, is also lighter in colour, and seems to be rather smooth. It possesses a blunt, rounded head with four large and elongated plates extending backwards, and pale beady eyes. Its mouth is full of sharp teeth, and below it is an elongated protrusion pointing downwards. Acnologia’s gargantuan, multi-layered wings are composed of the very same plates covering its whole body, which take on a formation reminiscent of a bird’s feathers. Its massive tail splits in two at its end, where the dark plates disappear, and takes on a double stinger-like appearance.

Personality
While the other Dragons appear somewhat friendly to humans, Acnologia is antagonistic towards them, and views them as insignificant. As Zeref puts it, it sees them as nothing more than insects pestering him.

History
Acnologia ran into Gildarts Clive when the Mage was on his "100 year quest," on Zonia mountain. In an instant, Acnologia took Gildarts's left arm, leg and an unspecified organ, leaving him to die before flying away, with Gildarts managing to survive with difficulty.

Tenrou Island arc
Acnologia heeds the accidental call of Zeref and attacks Tenrou Island. In the battle, it causes so much devastation that Makarov is forced to hold him off to allow the other Fairy Tail wizards to escape.