The Bone Dragon also sensed that something was not right with this situation. They weren't even trying to defeat it, but all the residents of this world understood how powerful it was.
The bone dragon had never been a living thing, it was a form of Nightmare Elemental, created entirely of the energy of this world, and the bone dust of fallen creatures. That was how the original Dragons in the other Planes had formed as well, purely of elemental energy and an aspect of the world that was overflowing and needed an outlet to help balance them, usually during the creation of the world.
It had formed much later than that, when the Lower Planes had lost their first war with the Upper Planes, and their army's bones had been shipped back, but it had also survived much longer, though it had been shattered countless times since then.