Used book – Buffy the Vampire Slayer, Vol. 5: The Warrior's Path, Christopher Golden, Nancy Holder

To each generation, its Slayer. Without her, vampires would have no match and would overrun the world. Today, it's Buffy... Between a math test and an educational visit to the Sunnydale museum, who would choose equations? Certainly not Buffy and her friends. Better to open your eyes wide in front of a traveling exhibition of Japanese art! Curious about everything, Willow reaches for a sword and cuts herself. From then on, she is no longer herself, gradually becoming a bloodthirsty monster! Buffy investigates... And discovers that the soul of a Chinese vampire was imprisoned in the sword.