Wow... way to spoil it for people who have the game but haven't gotten to that part yet...
Lol sorry, but that's like saying "I had no idea there were guns in Call of Duty". The main character (standard look) of Divinity 2 is even pictured on the front of the package as his human and dragon form . It's even on the list of features (like amazon.com):
"Fight as Both a Human and a Dragon - Don't settle for taming and riding a dragon when you can be the Dragon. For the first time, an RPG unleashes the very power of the Dragon on you. Climb high and vaporize all that stands in your way as you strategically use both your human and dragon forms to defeat Damian and become the ultimate Dragon Knight."
The fact that you turn into a dragon in this game was stated everywhere (magazines, internet, game package, steam info....).