The Crimson Realm (紅世 Guze ) is an alternate world in which Crimson Denizens and Crimson Lords originate. It is described as "an adjacent world which cannot be reached" (歩いて行けない隣 Aruite Yukenai Tonari ). Its original name is a word which means "Swirling Temple" or "Swirling Cathedral" (渦巻く伽藍 Uzumaku Garan ) in human languages. After the world was named the Crimson Realm by a poet in ancient times, Denizens began to use that name as well.
According to the Light Novels, there was a time where beings who originated from the Crimson Realm, as the Realm itself, bore no name. This was until long ago, a poet gave a name to the world, and called its inhabitants Denizens.
Originally, there are no possible means of going from the Human World to the Crimson Realm; a certain Crimson Denizen who rose up and became a Lord soon discovered a way to cross between worlds and it led the first group of settlers on the living world. The Denizens who arrived soon became selfish as they did not wish to give up a part of themselves and started to attack people for their Power of Existence, which is a primary source of nourishment for them in a seemingly unusual environment. This slowly led to the destruction of the balance of the world.
Because of this, a number of Crimson Lords decide to bind themselves in a contract between them and the inhabitants of the living world to become Flame Haze, whose main objective is to vanquish Denizens who destroy the balance.