The king told Alex about there family ritual .Firstly , Princess has a white Rooster. secondly at that time people thought to believe that white Rooster can see demon and Jin and helps to keep them away.So that's why white Rooster is highly respected by all people. So the ritual is that in order to get married the White Rooster also has to approve of it .
Alex know that this kind of thing could happen because he read about white rooster in that library. So he made a plan to kidnap princess white Rooster and replace this Rooster with his trained Rooster.