Huh. Assuming Unseeing Dragon is the correct translation (accounting for the lack of context), I’m gonna make a guess and say this king can literally blink away someone’s existence. That is, the king can literally choose to not see something. A bit of an overlap with the Truth Dragon, though, so not entirely confident in this.
That the Truth Dragon was beaten before and had their bloodline wiped is a plus for the Dragon Slayer Monks, though. Shows they’re not incompetent and that it was the combination of the Roar Bloodline’s fortress and the Wing Bloodline’s interference that stopped them from making progress.
The Full Moon Dragon title makes me think that their strength waxes and wanes like the phases of the moon, with their “full moon” equivalent being their strongest state, though, assuming this is correct, I wouldn’t put it past the author to give them a form equivalent to a “lunar eclipse” as well. And they’re the King of Scales because they shed their scales like a snake, I suppose.
I’m guessing the Undying Dragon is in agony or was turned mad somehow. Or devouring their bloodline is an attempt to transcend their current level of strength?