Owari no Seraph Wiki:

This novel is a prequel which narrates the series of events that occurred years before the start of the manga version of Owari no Seraph. It primarily tells the background story of what happened before the world became the way it is in the manga. Guren is the protagonist in this light novel series.

Both the light novel and manga started within a few short months of each other and are running concurrently at almost the same speed.