The concept of a Reality Marble is having one’s inner world materialized and projected onto reality following the World Egg theory. Generally, it is used to refer to the alien common sense of demons, in that it is how demons perceive reality and how Reality Marbles themselves are the natural abilities of demons and elementals. It is possible for humans to achieve this as an innate Bounded Field, though generally only in the cases of the powerful beings like the Twenty-seven Dead Apostle Ancestors and the greatest of magi who have spent years upon years researching. This ability is a similar and dissimilar method of “world creation” to Marble Phantasm, but rather than reshaping the world at their will, Reality Marbles allow only for the creation of a set world that defines the practitioner. They force all things to obey their rules, but it is limited only to that one pattern. It can be said that they can surpass Marble Phantasms at the same time, as they are capable of affecting things not of nature.
It is a magecraft that comes close to being real magic and, as a taboo among taboos and mystery among mysteries, it is one of the pinnacles for a magus for which research has been banned by the Mage’s Association. Those that do have a Reality Marble would inevitably earn a Sealing Designation if discovered, due to its nature as a unique magical ability that cannot be acquired through study and does not pass on to one’s descendants. Obtaining a Reality Marble is not something every magus can achieve through research. A magi far above all modern magi who could not utilize one could still construct “another world” of the same scale, with enough time and expenses.
The created worlds of Reality Marbles are completely cut off from normal reality, projecting their internal landscape and sealing off the regular World. Following the World Egg theory, the actualization of a Reality Marble is the result of switching the Self and the World, while keeping the boundary the same. The size of the Self and the World is switched, enclosing the World in a small container while the Self is expanded. The contained World is the World Egg and the name of the theory. They will encase anyone within range, causing them to disappear from view. Magi are still able to feel the presence of the Bounded Field, and while they cannot be directly affected from the outside, a large burst of mana, such as those from powerful Noble Phantasms, may be able to disrupt them. The World constantly attempts to crush the unnatural bounded fields, as they are not created by spiritual beings, meaning that it costs a great deal of magical energy to construct one and keep it stabilized. This generally only allows them to be used for a few minutes, but given their powerful nature, that is likely enough time to properly utilize them. Each world has its own set of patterns and rules, which can be said to be an invasion of reality by the diagram inscribed on the soul, and as it is just one aspect put into form, the user cannot add their own will into the form of the barrier.
In the case of Servants utilizing Reality Marbles, they will generally be limited to the Caster class, due to the spell’s nature as high thaumaturgy, but there are exceptions. Those of the Berserker class will likely never be able to utilize them, meaning that most who use them will be powerful magi with low combat abilities. It is possible for practitioners to make use of certain aspects of their Reality Marbles without activating them. Some can be activated inside the body of the user, although these are special cases.