the banished disappointing saintess is to marry the demon spirit count through a sacred marriage

Although Renisha entered the temple as a saint, she was unable to use her powers and was despised One day, the temple appointed her as a bride in holy matrimony. Renisha was happy that she could finally be of use to everyone, but... she was to be married to Welflem, a demonic count from the frontier who was rumored to be a cannibal. The purpose of the marriage was to be a sacrifice to seal Welflem's powers. For the sake of her hometown and to be of use as a saint, Renisha tried to fulfill her role as a bride, but Welflem refused to enter into a holy marriage

(Source: Funguild, translated)