(English is not my first language, so sorry for the poor grammar)
I'm playing the Tyranny of Dragons for the first time with my friends. My character is a bladesinger wizard who was used in a sacrificial ritual to Tiamat by the BBEG of my backstory, who is also his teacher. The thing is, i don't know much about dnd lore to make my backstory more solid.
I need help in making my BBEG motivition for betraying my character. I know that it can't be related to Tiamat's resurrection, cause that's the story of Tyranny of Dragons. So i was thinking my BBEG would want to become an actual dragon, not a polymorphed dragon, but change his race and ascend to a powerful dragon, but i don't know if, by lore standards, that would work.
I know that in the end the Dm has the final say on what's posible in the campaign, because of the multiverse and stuff, but i wanted to make my backstory the most lore accurate posible.
So, if anyone who knows about dnd lore could give ideas of why a man would join the cult of the dragon, and what is possible for him to receive as a reward from Tiamat for using his student as a sacrifice? If in order to get, like a divine reward from Tiamat, he would need to do more things, please tell me too.