Continual Flame is a low-level spell, that the *Entire Draketooth clan* had access to. Given that it's a human-built and operated dungeon, and that their *Primary* method of defense was illusions, it makes sense they'd want to illuminate the place. Both so they could see, and so enemies could see their illusions.

So this one, though it is caused by plot, at least makes a good deal of sense.