The dragon's backstory is way more fragmented than I expected, honestly. You get hints scattered across three books before the big reveal in the fourth, and even then it's told from the perspective of a blind priestess who might be an unreliable narrator.
It wasn't born, but forged from the grief of a dying star-god, which explains the star-metal scales and the fact it doesn't eat, but seems to consume silence or memory. The forging left it bound to the laws of a cosmic bargain, which is why it can't just burn the world but has to follow this ancient, cruel logic. Its 'origin' is less a birth and more a cosmic accident with tragic consequences.
I always thought the most chilling part was that it might not even want to be a destroyer. There's a passage where it observes a village it's about to obliterate and the text suggests a kind of weary recognition, like it's seen this play out a thousand times and is just following a script written into its very bones.