Prismatic Horn

You have bonded with a Prismatic Dragon in your lifetime and learned how to  cast spells from there teachings. The dragon has taken your horn and in return given you one of there scales, you make touch the scale in your spell book to transport one of your horns to the dragon in return of your other horn. You still dont understand why, but with your other horn comes a new set of spells you may cast.

After a spell switch the Dragon Scale goes dull until the dawn. Allows you to use a 2nd book of prepared spells once a day, however once you switch you must wait till dawn or take a long rest to recover this feat.