The Netherwrym is a undying demon is the form of a black dragon with flaming red horns. The Netherwrym is responible for the pyrexia that had struck Greede in WKC2. It was creating during the Dogma wars by the Yshrenian Empire as a weapon of war for use against the Athwan. There exists a bounty version of the Netherwyrm called Hades (Haltus in JPN direct translation) , this bounty is the hardest in the game with a recommened level of 70.
Even though it has a body of a dragon it is confirmed ingame by the dragon matriarch that it is infact NOT a dragon. This is in the dialogue of her during the retrosecticon trip for greed