Acnologia is the biggest, baddest, and most ruthless villain we have so far encountered in Fairy Tail.
An ancient dragon slayer who fought initially for coexistence in the Dragon Civil War 400 years prior to the start of the manga. However, he was consumed with bloodlust and so began killing every dragon he could, and bathing in their blood.
The result of this was that overuse of Acnologia's magic turned his body into that of a dragon's, and he started calling himself the dragon king and went on a rampage, earning the name The Black Dragon in the Book of Apocalypse.
"As a Dragon, Acnologia is mastodonic, and has an entire upper body covered in black, round, scales, which, in turn, are decorated by spiraling, blue markings. His lower body, specifically his chin, chest, stomach, inner tail, thighs, and arms, are gray in color, and appear to be rather smooth. Acnologia's eyes are white and beady, and his head is round and blunt with four, large, elongated plates extending backward. Acnologia's mouth is full of razor-sharp teeth, and, attached to his chin, is a protrusion that resembles an arrowhead. His large wings are feathery in appearance, akin to a bird's, and very similar to fellow Dragon Grandeeney's. The scales disappear at the end of Acnologia's tail, which, in itself, ends in a stinger-esque shape."
Even more badass is Acnologia's human form, which he chooses to revert to in the more casual encounters he has, or maybe just when he wants to travel under the radar. He has long flowing hair and dark skin, and he is tattooed or marked with the same kinds of markings that he bears in his draconic form.
Even with only one arm as a result of his fight with Igneel, Zeref still claimed that Acnologia could rule the world with the power he possesses. His motives are still unclear, though what is certain is that he labors for the elimination of all the dragons/dragon slayers.