Blasta_boy
The story starts where Ash and his Friends leave Krinna and head for Coumarine city, Lucario gained so much experience that he evolved pryer to his mega-evaluation, this form was believed to be a savage monster that ate human flesh and would kill anything in its way. Krinna however, still sees good in her best friends heart, they travel the Kalose region and try to escape the people form the Shalour gym, who may intend to kill Lucario.
BTW, if you read this book, I'd recommend watching Pokémon the series XY if you haven't seen it already, it will give you information that the book will not and things will make more sense.