Reality Marble

Intrinsic Bounded Field.
Originally referred to the alien common sense possessed by the beings called demons, but now also includes the unique boundary fields possessed by many others.
A subspecies of marble phantasm, it erodes reality and gives form to the user’s imagined world.
Unlike marble phantasm, however, the form of a reality marble cannot be freely adjusted. It is the manifestation of the user’s one-and-only self, after all, so appending their own will on top of its form is impossible.
On the other hand, since even things that are not part of nature can be influenced, perhaps it is actually even more amazing than marble phantasm, in a way.
Properly speaking, it is an ability unique to elementals and demons, but given enough time it is possible for some advanced practitioners to complete a magecraft to give shape to their personal imagined world and create a reality marble of their own.
Of course, if anything that isn’t an extension of nature like an elemental creates an alien world, the world itself will crush it. As a result, the upkeep of a reality marble requires a vast amount of energy, and most individuals are only capable of using one for a few minutes at a time.
Possessed by Dead Apostle Ancestors.
By the way, Nrvnqsr was able to escape the corrective influence of the world by using his reality marble within his own body.
Way to go, Professor Chaos.

Tsukihime Data Collection: Tsukihime Dictionary
§

① Intrinsic Bounded Field (Also see the entry for “Marble”)
② The name “marble” comes from glass marbles used in a well-known example of the clustering illusion.
③ A subspecies of marble phantasm. One’s inner world is given shape through world interference that erodes reality.
④ More specifically a reality marble is thought to be realized when a multitude of phenomena are interfered with probability-wise, forcibly invoking the impossible.
⑤ Originally the common knowledge of otherworldly being referred to as “demons.” Though a skill of fairies and demons, after extensive time and practice one can acquire the ability to give shape to images in your mind. A few of the top magi and the Dead Apostle Ancestors are capable of employing a reality marble.
⑥ However, objects created by a reality marble are at best still existences from another world, which means they are not the creation of spirits that are extensions of nature, so they are subject to being corrected or modified by the world.

Strength
Unlike marble phantasms, reality marbles can directly affect entities that are independent from nature (such as humans).

Weakness
A reality marble is at best a reflection of the creator’s predominant inner world, and cannot be changed freely at will. Thus, a reality marble is far less adaptable than a marble phantasm.

Colorful MOON Tsukihime: Tsukihime Dictionary
§

Intrinsic Bounded Field.
Originally referred to the alien common sense possessed by the beings called demons, but now also includes the unique boundary fields possessed by many people.
A subspecies of marble phantasm, it erodes reality and gives form to the user’s imagined world. It is the magecraft closest to magic, and a forbidden art within the Association.
Unlike marble phantasm, however, the form of a reality marble cannot be freely adjusted. It is the manifestation of the user’s one-and-only self, after all, so changing it with their will is impossible. On the other hand, since even things that are not part of nature can be influenced, perhaps it is actually even more amazing than marble phantasm, in a way.
Marble phantasm, which can change the world at will, but only has influence over elementals and nature. Reality marbles, whose form is limited in use to a single pattern, but force everything to submit to the user’s personal rules.
Both methods are similar yet different ways of changing the world, abilities that are extremely flexible yet limited in their own way.
…Properly speaking, reality marbles are only supposed to be used by elementals and demons, but given enough time it is possible for some advanced practitioners to complete a magecraft to give shape to their personal imagined world and create a reality marble of their own. Of course, if anything that isn’t an extension of nature like an elemental creates an alien world, the world itself will crush it. As a result, the upkeep of a reality marble requires a vast amount of energy, and most individuals are only capable of using one for a few minutes at a time.
It would seem that a majority of the 27 Dead Apostle Ancestors are capable of using reality marbles. Oh, and Nrvnqsr was able to escape the corrective influence of the world by using his reality marble within his own body. Way to go, Professor Chaos.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
§

Intrinsic Bounded Field. Originally referred to the alien common sense possessed by the beings called demons, but now also includes the unique boundary fields possessed by many people.
A subspecies of marble phantasm, it erodes reality and gives form to the user’s imagined world. It is the magecraft closest to magic, and a forbidden art within the Association.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary
§

A bounded field used to materialize and project one’s inner world onto reality. Rider’s noble phantasm “Army of the King (Ionioi Hetairoi)” is a reality marble.

Fate/Zero Animation Guide I: Glossary of the holy grail war I