Power of Existence (存在の力 Sonzai no Chikara ) is a fundamental energy required to exist in this world. It comprises practically every living and non-living thing in this world, and is also needed by Crimson Denizens in order to manifest themselves.
It is usually undetectable by humans unassisted and Crimson Denizens and Flame Haze are the only ones who can usually see Power of Existence, which usually comes in the form of a blue flame (anime only).
The Concept of ExistenceEdit
Power of Existence is essentially "how something gives an impact on its surroundings". It can be found on any object, be it living or non-living; it is also somewhat present on dead people or animals. This is because Power of Existence, like what its name implies, is the amount of existence something has in the world.
In a Normal Human's PerspectiveEdit
Humans are born oblivious to the concept of existence. They are born with a certain amount of Power of Existence; this amount increases or decreases depending on how influential they become. People who pass away or are killed by means other than being consumed by Crimson Denizens retain a small amount of Power of Existence, which allows other people to remember them even though they are no longer living.
Usually, a person who lives in a high-class society like royalty or the elite acquires a larger amount of Power of Existence because of the amount of influence of their respective families. Meanwhile, there exists "Great Ones" (偉大なる者 Idainaru Mono ) — people whose Power of Existence is very immense and can also be influential even when he/she does not undergo the process of becoming a Flame Haze. According to Alastor, Shana is classified as a "Great One", as she was able to survive the strain of casting the Tenpa Jōsai, a spell that killed its former caster (Mathilde Saint-Omer), as it requires a vast amount of Power of Existence. Alastor stated that the reason behind this is that unlike Mathilde, Shana was able to carry inside her a large amount of Power of Existence equal to that of what is required for the Tenpa Jōsai.
Status is usually part of the criteria set by Crimson Lords who seek their partners; Margery Daw is the daughter of an aristocrat, Wilhelmina Carmel is presumably a part of the royalty, Khamsin Nbh'w is a Middle-Eastern prince, Karl Berwald is a nobleman and Seere Habichtsburg is an aristocrat's famous entertainer.
In the Perspective of Torches and Flame HazeEdit
Torches display the effects of losing one's existence; as a Torch loses the fixed Power of Existence given to it, it loses its ability to give an impact to anyone it has interacted before becoming a Torch. When the "flame" inside it flickers out, it disappears, as if he/she did not exist in the first place.
Flame Haze, on the other hand, undergo a different process; when they agree upon the contract between a Crimson Lord, the entirety of the Power of Existence that he/she possesses will be taken up by the existence of the contracting Crimson Lord instead, causing the person to lose his/her identity. Khamsin has experienced this phenomenon when his father, surviving an attack from a Crimson Lord, lived on as if Khamsin was never his son.
Because of the innate ability of Power of Existence to create "something that exists" to anything that possesses it, it can allow an entity with temporary or no existence (like Rinne or artificial creatures created through other means) to live as if it was created naturally. Hecate once used part of her immense Power of Existence to create a vessel in her image, which she used as surveillance to Yuji.
Shana has also demonstrated that other beings can interfere with another person's (mainly a Torch's) existence. In Shana's case, she uses a small portion of the departed Torch's (Yukari) flame and melds it to hers. The entire process is not fully-explained, but the effects are extensive and immediate, causing Shana (or any Flame Haze for that matter) to exist with the identity of the Torch.
Likewise, Crimson Denizens can do the same thing, as the Crimson Denizen Mare and the Crimson Lord Leanan-sidhe use the existence of Torches (the "Mystes of the Papagena" and Lamies respectively) to conserve their consumption of Power of Existence and to conceal their presence to others.
Making the Impossible PossibleEdit
Power of Existence is the main requirement for Unrestricted Spells; it goes both ways: taking Power of Existence from themselves or taking the Power of Existence of things on his/her environment.
While it is not necessary to give away the Power of Existence to create Treasure Tools, some humans, such as Tenmoku Ikko and Johann, intentionally sacrifice their own Power of Existence in order to become the Mystes by their own will.