The boss animated a rock in the caverns into a lovable corgi named Pebbles. Afraid of changing back to his lifeless rock form, he does his master's bidding to help ensure there is enough magical power in the caverns to keep him animated. If all goes well with her plans to restore magic, the master promised that she will transform him into a real dog.
Each mastery level adds another card to the cardpool for defeating Pebbles. Each of the bronze, silver, and gold mastery levels unlocks a new card, while it takes all three diamond and onyx levels to unlock a single card.