Origin and magical inheritance
Very little is known about Clow Reed and his past. He was born to a Chinese woman and an English man. Both of them were extremely powerful magicians, and because of the mix of eastern and western magic he recieved in his blood, Clow ended up becoming one of the most powerful magicians in the world.
Back in his day, anybody could do basic magic if they followed the recipe, but what made Clow special was that he used his powers to create a new form of magic: the Clow Cards. He also created two guardians, Keroberos of the Sun and Yue of the Moon.
After living hundreds of years, Clow decided it was his time to end his life in this world, and began preparing for who, in the future, would become the master of the Clow cards and Kero and Yue. He sealed the cards within a book and closed the seal. He then placed Yue's soul on the back and Kero's soul on the front, and then... he died.
Hundreds of years later, Sakura ended up uncovering the Clow Book, releasing the cards and awakening Kero. Unknown to her, Yue was hiding in the form of an older boy, Yukito, who Sakura had strong feelings for (Yukito knew nothing of his powers or Yue).
After Sakura sealed away all the cards, Yue revealed himself and began the Final Judgement. Sakura defeated Yue, transformed her staff and became the new master of the Clow. After the battle, Sakura was transported to the astral plane... where she met... Clow Reed.
"Sakura." he said, "Look after Keroberos, Yue, and all the cards for me."
Sakura tried communicating with Clow, but as this was an astral message and Clow was dead, he never replied.
"Your staff contains a new power, Sakura. Not of the sun or the moon... but the power of your own star. I fear I might make things difficult for you, but Im sure you'll be alright."
So Sakura thought everything was over, until a new boy, Eroil, started at her school. Unknown to her, he was Clow Reeds reincarnation, and possessed all Clow Reeds abilities.
Eroil kept his magical powers a secret to Sakura, but still became a loyal friend to her. Behind her back, he would make magical incidents happen, which would force Sakura to use her newfound Star Magic to transform the Clow Cards into Star/Sakura cards.
After Sakura had transformed 15 of them, Eroil confronted her, Kero and Yue. He also had two guardians, Spiniel Sun and Ruby Moon. Eroil cloaked the world in darkness and put everyone to sleep, and told Sakura that if she was unable to transform the right card in time, then everybody would stay asleep forever.
Using fire attacks, Eroil managed to give Sakura's magic a boost as she transformed the Watery and Shield cards. After she realised she needed to use Light and Dark to sort out the darkness, Sakura transformed them and ended Eroils spell.
Fujitaka and why the Cards were made
While Sakura questioned Eroil about why he caused the incidents, her father, Fujitaka appeared. Eroil said that his magic couldnt affect Fujitaka, because he was Clow Reeds other reincarnation (who didnt recieve any magic or memories).
Eroil told Sakura that only she could cast a spell to split the magic and memories and to end the Clow Legacy. She did, and her father inherited half of Clow Reeds magic. Sakura was now the strongest magician in the world.
Clow Reed wanted this. He made the Clow Cards because he knew that one day, if he did make them, a young girl would end up possessing more power then he did and would stop him from possessing the strongest magic in the world (which he thought was a burden).