Acnologia

Acnologia is one of Fairy Tail's overarching villains, the "Dragon King" who nearly wiped out the dragons in Earth-land's Dragon King Festival. Once a human Dragon Slayer who hated dragons for killing his family, he became an immortal black dragon as a side effect of his magic and can switch between human and dragon form at will. His Dragon Slayer magic has no element, which makes him immune to all magic. For four centuries he roams as a bringer of destruction.

He nearly destroys Sirius Island after Fairy Tail's fight with Grimoire Heart, loses an arm killing Igneel, and joins the war between Ishgal and the Alvarez Empire to end the dragons by killing the surviving Dragon Slayers. Trapped in a "space between time" by Blue Pegasus and Anna Heartfilia, he eats his way out and gains godlike power, his soul splitting from his body. Lucy seals the body in Fairy Sphere and Natsu Dragneel kills him by destroying his soul.