Marble Phantasm (空想具現化, Kuusou Gugenka?, Manifest Fantasy) is the ability to materialize a phantasm, a vision. It is the ability of an Elemental or True Ancestor to connect their will to nature, to interfere on probabilities, and to transfigure the surrounding world at will according to their vision of the world. As the user is a part of nature, he or she can change the world as desired, but in the end, what an elemental can act on is only something within the scope of nature. The capacity of the actualization is proportional to the scale of the elemental. The term “Elemental” usually refers to spirits from nature when used by itself.
Transformations independent from nature, such as transforming a human, cannot be realized, unless done indirectly. While on the other hand animals can be controlled by the Elemental. The spiritual realm isn't a world separated from the real world, but localized regions thought of by the elemental that creates them; hence it is possible for a human to stray into it.
When Arcueid uses Marble Phantasm on Michael Roa Valdamjong, she turns layers of the atmosphere in the corridor into a vacuum that cut him like a knife would a radish. Although Roa himself was not affected, there was no way to avoid being chopped in pieces by the atmosphere, and Roa only narrowly managed to remain as just his ankles - Arcueid's imagination didn't go as far as having the floor dislocate. When not restricted, Arcueid can create a village in the mountains. In locations where humans have made changes, it will take more time to first break it down.
The concept works on "clustering illusion" theory of probability. Much like a mathematical metaphor involving drawing a white marble from a bag of black marbles with a one percent chance, Marble Phantasm is the ability to raise that chance to one hundred percent and definitely pick the white one amongst all the black ones. This remains within the laws of the world, but similarly, a Reality Marble would allow for all of the marble to turn into white ones. This is something the world wouldn't naturally allow. It can be considered more efficient, but Reality Marbles can only specialize in that one aspect, while Marble Phantasm can perform any number of things. Reality Marbles can also affect things not of nature, so they can also be called superior in that sense.
Millennium Castle BrunestudEdit
The Millennium Castle Brunestud (千年城ブリュンスタッド, Sennenjō Buryunsutaddo?), originally the lair from Crimson Moon Brunestud and a piece of his Reality Marble, is currently reproduced by Arcueid's use of Marble Phantasm. The original master of Millennium Castle has already been annihilated, but since then, those True Ancestors that were able to form it, through their Marble Phantasm ability, were given the title of Brunestud and treated as royalty. True Ancestors, beings that have no concept of social stratification, call Arcueid the princess for this reason. It is existing in the location closest to the moon, not topographically, but where the world receives the most influence of the moon. It is the castle Arcueid sealed herself into, depicted as chaining herself to the throne, after getting tricked into drinking blood by Roa and killing other True Ancestors. Even without Arcueid around, the castle is still seemingly in existence, but without "life". To the True Ancestors, the castle was probably their absolute protection.
 Tsukihime Dokuhon PlusPeriod - TSUKIHIME Dictionary: Marble Phantasm [Unusual talent], p.178 English | Japanese
Marble Phantasm [Unusual talent]
Imagination Realization. As the name suggests, it is the ability to manifest anything you can imagine.
It is the ability possessed by elementals, which are the sense of touch of the natural world. By connecting their will directly to the world, they are able to transfigure the world into whatever environment they imagine. Of course, the level of manifestation varies depending on the scale of the elemental, and it goes without saying that only nature can be transfigured – things that exist independently of nature are immune.
In Tsukihime, Arcueid eliminated Roa by using marble phantasm to turn the layers of the atmosphere in the hallway into a vacuum, like a carving knife slicing up a daikon radish. Though she could not affect Roa himself, it was impossible for him to avoid the finely sliced atmosphere, and he was reduced to nothing but ankles.
(The gap above the floor was the result of a deficiency in Arcueid's imagination.)
Also, in the world of Tsukihime spiritual realms are not alternate dimensions, but rather exist within the world itself. Things like legendary lands and countries of eternal spring are the products of the imagination of whatever elemental rules over them. Consequently, it is possible for humans to wander into them by accident.
The castle in the mountains that Arcueid calls home is actually a product of her marble phantasm.
 TYPE-MOON BBS English | JapaneseMarble Phantasm marble explanation
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