The Immortal Demon

"Demons." Ageless, immortal beings living in the shadows of the mortal realm. The way to gain immortality is to kill a "Demon"; the price is to take on their role. A man who killed his Demon lover; a Demon who commits suicide over and over without someone to kill her; a girl who interacts with Demons.... The curtain rises on a life-and-death stakes game of musical chairs between Demons and mortal humans.

(Source: MangaPlaza)