Outline
Sakura comes down with a fever that prevents her from going to school. Despite the efforts of both Touya and her father, the fever doesn't go down. In a dream, Clow tells her that she needs to return to her guard state in order to preserve energy. Instead of doing that, Sakura senses that something is wrong with Tomoe, escapes out the window, and visits her friends, only to find that she's been possessed into attacking Sakura.
When the cards saved Sakura's life, another card was made. This card was a Dark Sakura with black wings and the ability to control others through Pride, Greed, Wrath, Lust, Envy, Sloth, and Gluttony. The name of the card was Sin.
Sin forces Touya, Shaoran, and Yue to hurt Sakura after Tomoe is knocked out. Touya tries to fight it, but he ends up breaking Sakura's leg, and Shaoran knocks himself out rather than hurt her any further.
Basically, Sakura talks to Sin, tells her to value her own worth, since Sin was feeling as though she was no more than Sakura's shadow. Once convinced, Sin, moved by Sakura's compassion, sacrifices her life to save Sakura's.
The End