Madoromi no Komoriuta

When people fall asleep, they generally see dreams. But when a person's heart is burdened with darkness, it can cause demons to emerge. Demons which feed off of and taint the hearts of their victims. This is a story about a young man who is talented at singing and a man with insomnia, who are members of an organization tasked with slaying such nightmare causing demons. (Source: DLsite)