Greedy, Vain and Deadly
A huge, green, winged lizard that spews poisonous chlorine gas.
About a hundred years ago this dragon showed up with its army, conquered the Vale, drove out most of the inhabitants and enslaved those who stayed behind, and generally made life miserable for everybody in the region. Maybe it wanted something specific or perhaps it was simply seeking its own territory, as dragons are wont to do.
Recently Lord Mark subdued and captured Vermithrax. How exactly he accomplished this remarkable feat is not known. Vermithrax is, at least in theory, no longer a threat, but clearly finds its current predicament intolerable.