Acnologia, also known as The Black Dragon and fearsomely reputed as The Dragon King, was a cataclysmically powerful Dragon Slayer that can take the form of a Dragon at any time due from his overuse of Dragon Slayer Magic and bathing in the blood of the Dragons he killed. He was referred to as Doctor in his early life that took care of Dragons but due to them murdering his family, destroying his city, and "claiming" the life of a child, he took it upon himself using the name of the Dragon whom he despised to slay every Dragon in existence, believing they only cause destruction and pain.
And he is reprised by J. Michael Tatum in the English dub and Kohsuke Toriumi in the Japanese dub.