Dragons there are that roam the Urth and that take to its skies.  But among the most loathsome and fell of dragon kind are the vile, poisonous wyrms that inhabit the darker recesses and caverns that lie deep underground.  Some of these loathsome black wyrms can grow up to twice the size of their fire breathing cousins and have never seen the light of day, for centuries content to bloat themselves upon cities of the goblins, elves and dwarves carved out miles below Urth’s surface.  Rarely encountered above, still such monstrosities can, and will, surface if sufficient hunger drives them, or if stirred to depredation by powers even greater and more evil than themselves.