Before entering Zeno Castle, go to the Nature continent and go to the Center of the World. You will be asked to enter a secret area where you will encounter Shigenobu Matsumoto, an artist for the Duel Masters Trading Card Game. You will be required to duel him. After winning the duel, he will ask you to trade in 10 extra cards for 1 card of his own. The rarity of the card he gives you will depend on which cards you trade in.