Marble Phantasm

Imagination Realization. As the name suggests, it is the ability to manifest anything you can imagine. I really wish I could do this.
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.

Tsukihime Data Collection: Tsukihime Dictionary
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① Imagination Realization. (Also see the entry for “Marble”)
② The name “marble” comes from glass marbles used in a well-known example of the clustering illusion1.
③ The user directly connects and imposes their will on the world which creates an interference that transforms the world as they see fit (as they imagine). They make reality an illusion (something impossible in reality).
④ More specifically a marble phantasm is thought to be realized when a multitude of phenomena are interfered with probability-wise, forcibly invoking the impossible2.
⑤ Anything can be created as long as it's a possible result of an interference in the probability of (natural) phenomena.
⑥ In Tsukihime, a Marble Phantasm is the materialization of a part of an imaginary world which results from the recomposition of the imaginary world by spirits so it can exist on Earth. Therefore, it is possible for regular humans to wander into a Marble Phantasm.3

Strength
The Marble Phantasm user can transform the world around them at will. It is exceedingly versatile.

Weakness
Only nature can be transformed by a Marble Phantasm. A Marble Phantasm is incapable of directly affecting existences independent from nature such as humans. Accordingly, in order to affect an independently existing individual/phenomena, the creator of the Marble Phantasm must first create an object in reality/nature, and then use it to affect the target. The effects of world transformation are not directly applied to the target.

Colorful MOON Tsukihime: Tsukihime Glossary
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A special attack performed using Arcueid's special gauge (magic circuit). Also called Marble Phantasm. In Tsukihime, Arcueid's marble phantasm is depicted as an ability that allows her to materialize her thoughts. In Melty Blood, TATARI (Night of Wallachia), who transforms into Red Arcueid uses Marble Phantasm to materialize Millenium Castle Brunstud. Originally, Marble Phantasm was a power held by elementals, or “the touch of the Earth,” but it turned out that there are some within the Dead Apostles who can also use it.

MB Official Strategy Guide: Melty Blood Glossary
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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.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
Translator's Notes
  1. ^ Clustering illusion is when you find “non-random” patterns or groups in actually random sets => Also see the entry for “Marble.”
  2. ^ This means that the Marble Phantasm effectively creates a continuous chain of desired coincidences through by manipulating the probability of an event, to realize what is almost a complete overwrite of the world in the Marble Phantasm area. See “Marble.”
  3. ^ Spirits are of nature, so the world doesn't reject a Marble Phantasm, which is a major point of difference from a Reality Marble.