"Reality Marble" originally, this was the otherworldly common sense possessed by beings called demons, but now it refers to unique bounded fields possessed by many kinds of beings. it is a subspecies of materialization of fantasy, a bounded field that gives form to the user's inner world and lets it erode reality. what differs from materialization of fantasy is that its form cannot be freely decided. since the caster's one and only inner self becomes its shape, the caster cannot impose their will on the form of the bounded field. however, conversely, because it can affect even things that are not natural, it may be said to surpass materialization of fantasy. originally it was an ability only for elementals and demons, but over many years magecraft that gives form to an individual's inner world was completed, allowing some high-ranking magi to form reality marbles. however, if something other than elementals, which are extensions of nature, creates an otherworld, the world itself moves to crush that different realm. therefore maintaining a reality marble requires enormous energy, and as a result an individual's reality marble lasts only a few minutes. dead apostles, being ancestors, are capable of reality marbles. incidentally, Nrvnqsr Chaos had made the inside of his own body into a reality marble, thereby escaping the world's corrections.

These settings were established before the release of Fate/Stay Night (2004), so some details may differ slightly from the current canon.

The official type-moon glossary entry for "Reality Marbles". TL by me.