GARO (2005)

Kouga Saejima, a Makai Knight that holds the title Golden Knight Garo protects humanity against "Horrors", dark demonic creatures that possess bodies to lure and devour humans. One night, he saves a young woman named Kaoru Mitsuki from a Horror, but in the process she gets stained by its demonic blood. As a rule, a person that has been stained by a Horror’s blood should be cut down (killed) by a Makai Knight to avoid an unbearably painful death in 100 days, despite this rule, Kouga spares Kaoru’s life and vows to purify her before time runs out.