Reality Marble

Intrinsic Field.
Originally referred to the alien common sense possessed by the beings called devils, but now also includes the unique boundary fields possessed by many people.
A subset 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 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.
Properly speaking, it is an ability unique to elementals and devils, 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 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 subtype 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 Originating from common knowledge of the “demon kind.” 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 a few of the ② Dead Apostle Ancestors are capable of employing a reality marble.
⑥ However, objects created by a reality marble are at best still objects from another world, which means they are not the creation of a spirit or 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 Glossary
§

Intrinsic Field.
Originally referred to the alien common sense possessed by the beings called devils, but now also includes the unique boundary fields possessed by many people.
A subset 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, which are limited in use to a single pattern, but force everything to submit to the user's personal rules.
Though the methods are different and they are each limited in their own way, the fact remains that they are both powerful abilities that change the world.
…Properly speaking, reality marbles are only supposed to be used by elementals and devils, 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.
Most 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 Field. Originally referred to the alien common sense possessed by the beings called devils, but now also includes the unique boundary fields possessed by many people.
A subset 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