This was one of the first songs to be written for Imaginaerum and was described by keyboardist and composer-in-chief, Tuomas Holopainen, as, "a duel between the Devil and Mother Gaia." (Mother Gaia is the goddess who the ancients believed represented Earth). He explained to Metal Hammer magazine: "Since this is a dramatic album about the celebration of life and existence, you need the dark side as well. I love the dark side and I love all kinds of evil stuff, so we needed a few songs to describe those. 'Ghost River' and 'Rest Calm' are the two songs that are talking about the dark side of life."