BLOOD-C

Saya Kisaragi is a high school student living a dual life. By day, she's a typical student, but at night, she wields a ceremonial sword to defend her village against supernatural threats. The sword was given to her by her father for this purpose, and she's been successful in eliminating the monsters that appear. However, Saya's world is disrupted when she overhears the monsters discussing a broken covenant, a term she's unfamiliar with, and a mysterious dog appears, questioning her obligations to protect the village.