The Guardian Dragon and the Fake Princess - Chapter 15

The author describe to be the pact maker for dragon, they are required 3 things… their blood, their are to named and lastly, the purity of their soul. So just not anyone within imperial family able to become one as long this 3 condition arent met but then again… how the f the emperor manage to be one? Does he forcefully pact the dragon? Its quite obvious that the third requirement is not met.