Garo's first season follows Kouga Saezima, the current Golden Knight Garo as he protects humanity against dark demonic infestations called Horrors. The series kicks off when he meets a young girl names Kaoru whom he saves from a Horror, but gets stained with Horror's blood. The Makai Knights have a rule that those that have been stained by the blood of a Horror shall be executed before experiencing a painful death after approximately 100 days.
[Series summary from IMDb]