CLAMP and MLP:FiM crossover.
Twilight Sparkle discovers a strange book from her uncle's collection, and accidentally releases a powerful force known as the Clow Cards. Now, she must become Cardcaptor Twilight, and recaputre all of the cards, before a great catastrophe befalls the world.
Meanwhile, Yuko Ichihara has spotted her chance to finally claim the prize she has always sought from her biggest rival, but is unable to capture the Clow Cards herself, due to certain limitations in her powers. Therefore, she leaves it in the hooves of one of her customers, who, like Twilight Sparkle, carries the blood of Clow Reed in her veins.