Prologue: Yin Xan

As the mysterious Clow Reed spent his life in peace with his guardians and Clow Cards, a young, teenage sorcerer was developing his own magical abilities. Yin Xan was of both Chinese and Japanese descent and loved magic more than anything else in the world. He attended a reading performed by Clow Reed who used the Clow Cards to tell a child's fortune. Impressed and inspired by the cards, Yin became acquainted with Clow Reed who showed him a few demonstrations of the cards' true power. Kerberos and Yue did not think this wise; they felt that something was off with the man, as if he was not entirely balanced. Nevertheless, they kept their thoughts and feelings to themselves.

With due haste, Yin returned to his home and endlessly studied his heritage and the magic of Japan and China. He was fascinated with the concepts of the Yin Yang and the elements of all the world's beliefs: water, earth, air, fire, wood, aether, metal, void, ice, and thunder. It took him ten years to come up with conceptual designs for the theme of the pair of his own guardians and deck of cards. Appearance, abilities, weapons, and even possible assumed forms were all concepts of Yin's works.

At long last, he finished the designs and created a guardian beast ruled by the moon named Zaigou and the creature's counterpart, a humanoid guardian of the sun, that the sorcerer called Dishi. Zaigou represented the Yang whereas Dishi represented the Yin. For many more years, while he was steadily decreasing his lifespan to create his own deck of cards, he developed a strong bond with his guardians. He did not need to teach Dishi much because she developed into a very friendly, competitive person with a natural knack for expressing herself. Zaigou was another matter. Though he preferred an animalistic form, Yin often placed Zaigou in assumed human forms to "train" him in social matters. By means of books and experience, the reserved guardian learned manners, respect, honor, and simple communication. Unfortunately, he still seemed incapable of extreme emotions and was completely unaware of how to handle them, including people who liberally and passionately expressed all areas of the emotional spectrum.

Soon after creating twenty-six harmonic cards, Yin Xan searched into the future to choose a successor. During the time he was foreseeing his future, the man had discovered a series of terrible events. There was very little he could do to prevent them, but he saw the light that could emerge from the darkness and a choice that he could make to give this light a chance. His successor was the light among the darkness, but he kept the identity a secret from his guardians. Hours later, he did the one thing he could to prevent the world and the future from falling into everlasting darkness: he shared a wish with his guardians. He wanted Zaigou to select a candidate not by his magical ability, skills, or training, but instead by the amount of light within his heart. Yin Xan knew that Zaigou would never be able to see the light he was talking about, but that he could sense it due to his connection with the Yang. No matter what was to come in the future, he told Zaigou, the guardian was to keep his promise and select the one with the most love, the most hope, and the purest heart to become the candidate for the new master of the card deck that Yin would one day complete. As for how fit the candidate was in the "measurements of the heart", Yin decided that Dishi would be the one to test him in ways that she saw fit. Both guardians promised their master that they would perform their duties with all their hearts. It was that very night that the wheel of fate set itself in motion…

Not long after he spoke his wish aloud, the sorcerer went to visit his idol in order to show the man his progress. When he got there, a member of the Li clan informed him of Clow Reed's passing. After the funeral, Yin Xan returned home. Over the next two years, he began spurting out new cards. All of the violence, rage, and insanity that filled him seeped through his magic into each of the next twenty-six cards. His lifespan decreased twice as much due to the speed at which he was creating these new cards. Whenever Dishi or Zaigou tried to talk sense into him and return him to his old self, he would burst into tantrums with violent magic. His aura completely changed. There was nothing left of their master's old self. He had transformed into a monster of complete darkness, almost a demonic being.

At long last, the deck was completed. The sorcerer would have tried to destroy the world with his deck, but neither of his guardians wanted that to happen. They fled and returned shortly after with a powerful miko-sorceress that specialized in exorcisms and calming demons: Inagi Sen. Even though these specialties could not cure his insanity, she managed to seal away his rage and violence that took the form of a final card. By the time she had done this, however, it was too late. The man's magical tantrums had stripped him of all but a few months of his life. On his deathbed, he regained some of his old self. Once he thanked the woman, he created a book called "The Yin" to store the cards within. This took up most of what was left of his lifespan, but the man's final magical act was, after he said his goodbyes, sealing his guardians within the front and back covers. With his dying breath, he entrusted the woman, Inagi Sen, with The Yin. It was kept safe until the chosen day…