Yu-Gi-Oh! Stay Night is a crossover fan fiction between Yu-Gi-Oh series and Fate Stay/Night. It is created by a fan during Yu-Gi-Oh! Zexal era. The story is a spin-off of Yu-Gi-Oh 5D's and take place two years after the series end.
The story take place in fictional city Pastele, the city of art and epic poetry located in northern Bucharest, Romania. Many years ago, the Goddess Calliope gather many stories of heroic deed from all around the world and harvest their soul. Heroes after heroes were sealed inside a stone tablet for duel monster and place inside Valhalla, Hall of the Fallen. With so many souls of the heroes under her collection, she decides to do something about it. She wants to create more epic stories so she can reap the soul of the heroes when they died. So, she explores the memories of fallen heroes whose soul is collected. Then, she found the Holy Grail inside the memory of King Arthur. With her power, she transform the memory object into reality and declare that any people whom she choose who can win the grail through tournament will be granted any wish but she didn't tell them about what happen to their soul when they died.
Many families from around the world has been chosen by Calliope. Most of these family ancestor is a person with strong magical/spiritual power. Yukia's family ancestor is a powerful wiccan witch while Shiro's ancestor is a wise Shinto priest. People with this gift favored by Calliope as heroic magical soul is more "precious" than the non-magical one. They will be taught the method to summon their own Heroic Spirit card that can be used in battle against the other chosen. The summoned Heroic Spirit is referred as Servant while the summoner is referred as Master.