Onox the General of Darkness starts the game by kidnapping the Oracle Din, and takes her away to his castle and imprisons her in a giant crystal. He then faces off with (and is later defeated by) Link in two grueling battles; the first of which in his human form, and the second in his true form, that being The Dark Dragon. Unbeknownst to everyone, he is merely a pawn of Twinrova in a scheme to resurrect Ganon. His stage is Onox's Castle, and as stated before, he possesses two forms, the first being that of a heavily armored knight, and the second being that of a massive dragon. His first form's main method of attack is swinging his ball and chain weapon. In his dragon form, his main weapons are his massive claws and fire breath. It should be noted that Onox may share some relation with the Iron Knuckles, as they share similar armor designs.
Source: Wikipedia, "Characters in The Legend of Zelda series," available under the CC-BY-SA License.