To Bahamut, he charged with teaching the reptilians wisdom and light. And to Tiamat, he showed her all the inhabited worlds of the Great Beyond and to her he charged to lead the reptilian spirits to conquer. Both Bahamut and Tiamat agreed to their duties and then Io set about creating.
to some spirits, he infused with the elements to give them power. He gave them the body of great winged beasts and the ability of speech and reason. These he called the dragons. To others, he infused with elements, but gave them the forms of humans. He created both male and female, so that they may reproduce. These, he called the dragonborn.
As the dragonborn continued to multiply and reproduce, some followed Bahamut on the path of Wisdom. While others followed Tiamat on the path of conquest. Tiamat granted the dragonborn knowledge to build technologies so they can achieve their purpose. And when the dragonborn advanced sufficiently, they started to build their great ships to conquer the other inhabited planets of the Great Beyond. In time, there was a schism and the dragonborn race became divided. The ones that followed Tiamat left and went into the great beyond, while those who followed Bahamut stayed and followed the path of wisdom to asscension into dragons.
And this is the creation myth of the Dragonborn.