I guess this could work... it just seems a little anti-climatic.
FD was very specific about there not being any other dragons in the Vale.
Cadran's extraplanar ancestors have intermingled with the greater human populace ever since they fled the Dragon War. They've done their best to fit in, fade into the background. They were successful merchants for the most part, with a handful of sages and mages thrown in.
After falling out with his family, Cadran studied as an Adept in the Magistry. There, he never really got into the full spirit of worshiping Valkran, but he persevered nonetheless. For seven years, he studied magic and worked in the temple's library. After a particularly long day of reviewing the records of an expedition into the Brackenreach, he fell into bed exhausted.
That night, he dreamt that he was in the Brackenreach with the expedition. Night was falling, and the group was setting up camp. A twig snapped in the shadows, and everyone went for their weapons. Two glowing eyes shone in the last gleam of the day's light. Slowly, one by one, four more pairs of eyes opened. From five giant maws, a high pitched cackle shook the party, and Cadran woke in a cold sweat. Shaking it off as a bad dream, he organized the stacks that day, doing his best to forget the cackle.
As soon his eyes closed that night, he was thrown back into the dream. Great jaws peeked out from the underbrush, and let loose blasts of acid, flame, frost and electricity. In seconds, nothing was left standing, except for Cadran. He felt the giant shadow of a wing pass over him, and the heads roared to sky, shooting energy into the sky. When he followed the blasts to their target, there was nothing there. This mystery was quickly solved when a great wind nearly knocked him off his feet, and the ground shook behind him awake. Instead of the terror, he felt... secure. Strange.
The next night, he returned to the Brackenreach in his mind. From the foliage emerged the heads of five dragons, attached to a single body. Looking behind him, Cadran saw another dragon, this one the color of platinum. The dragon let loose a roar that caused the air to ripple as it passed. The center three heads of the chromatic dragon whipped back, as if they were rubber. With a growl, the one he had identified as Tiamat, took to the skies. And if that was Tiamat, then...
Cadran dropped to his knees, bowing his head low, but Bahamut only let out a deep chuckle. With the chuckle, a sense of immense love washed over him. "Would you like to join me?" Looking up, Cadran saw not a dragon, but a kindly old man, extending his hand to the Lesser Aasimar.
Cadran took the hand.
He was suddenly more awake than he had ever been, regardless of the fact that the moon hung straight overhead. He quietly rushed to the library, where he scoured the stacks for a book he once filed away, but had never before given a second thought. He found it open on a reading table, with the moon shining directly on it, providing just enough light to read. After silently cursing the wizard who didn't return the book, he dove into "The Dragonborn of Bahamut", by Kall Furth. It laid out his next steps to become the son of the Platinum Dragon, so Cadran gathered the belongings he could carry, and filed his resignation.
That dawn, he had prayed for the first time to Bahamut, and felt invigorated. He could wield such a greater array of spells than he ever could when it was Valkran giving him the spells. But the Ritual of Rebirth called to him, and so he set off to ask Palax for a few of her scales. The fates, or more likely Bahamut, were smiling on him, so when he
Having dropped his family name when he joined the Magistry, Cadran took the name Tiburux or "Born as Platinum", in honor of his new father.
Meant to get to this earlier. For fair warning, I'm probably going to go for my weapon the first time we meet, because Kobolds often associate with Chromatic Dragons.