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The concept of a Reality Marble (固有結界, Koyū Kekkai?, innate bounded field) is having one's inner world materialized and projected onto reality following the World Egg (世界卵, Sekai Tamago?) theory.[1] Generally, it is used to refer to the alien common sense of demons (悪魔の持つ異界常識, akuma no motsu ikai joushiki?) and that it is how imaginary demons perceive reality and how a Reality Marble itself is their natural ability. But 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 allows 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.[2]

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.[3][4] 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.[5] Obtaining a Reality Marble is not something every magus can achieve through research. Caster, far above all modern magi, cannot utilize one, but she could construct "another world" of the same scale with enough time and expenses.[6] Areas that are full of grudges, filled with remnants of strong regrets, are like Reality Marbles to spirits.[7]

The created worlds 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 Excalibur, 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, even the strongest Dead Apostle can not maintain a Reality Marble for more than a single night. 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.[2]

In the case of Servants utilizing Reality Marble, it will generally be limited to the Caster class due to the spell's nature as high thaumaturgy, but there are exceptions like Rider.[8] 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.[9][10] It is possible for practitioners to make use of certain aspects of their Reality Marbles without activating them. Unlimited Blade Works is the responsible for the Projection and Reinforcement abilities of its practitioners, and Rider is able to summon a single soldier from Ionioi Hetairoi at any time without bringing about the corrective impulses of the World. Some can be activated inside the body of the user, such as Unlimited Blade Works going out of control and creating swords inside of Shirou Emiya, and in a special case, Nrvnqsr Chaos only deploys his Reality Marble inside his own body, which allows him to avoid the corrective impulses of the World.

The mechanics of a Reality Marble being activated when one is already established is unknown. Within SE.RA.PH of the Moon Cell, all areas are manifested as Reality Marbles, and they can be overwritten by those of Servants. Within Fate/Extra CCC, Archer's Unlimited Blade Works can be overwritten by Archer's own Unlimited Blade Works, Caster's Eightfold Blessing of Amaterasu, and Saber's Aestus Domus Aurea. It is unknown if this reflects an actual occurrence or if it is limited to a gameplay feature.