In Japanese, "Youmu" (妖夢) can translate to "Ghostly Dream" or "Phantom Vision." This is a popular term in modern Japanese media (such as the Touhou Project ) to describe beings that exist between the physical and spirit worlds.

Youmuin didn't hunt for gold or secrets; he hunted for the things that lived in the back of your mind when the lights went out. He was a "cleaner" for the soul, a specialist who walked into the jagged landscapes of people's worst fears to drag the darkness back into himself. The Client