Marble Phantasm

As the Japanese name implies (空想具現化 Kuusou Gugenka), Marble Phantasm is the ability to materialize a phantasm, a vision. Essentially, it is the ability to transfigure the surrounding world at will according to the vision a spirit has of the world. In the end, what a spirit can act on is only something within the scope of nature. A transformation independant from nature cannot be realized.

In the story, when Arcueid uses Marble Phantasm on Roa, 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.

In the settings of Tsukihime, the spiritual realm isn't a world separated from the real world, but localized regions thought of by the spirit that creates them; hence it is possible for a human to stray into it.

The castle in the mountain area that Arcueid lives in exists from her Marble Phantasm.

Source: Tsukihime Dokuhon PlusPeriod dictionary entry (p.178)

The English name comes from a metaphore. When drawing a marble in a bag of black marbles with a 1% chance of drawing the one white marble, Marble Phantasm is the ability to raise that chance to 100% and definitely pick the white one amongst all the black ones; this remains within the laws of the world. As opposed to this and following a similar metaphore, a Reality Marble would be the ability to turn all the marbles into white ones, which is something the world wouldn't naturally allow. While this can be considered to be more efficient, this is, however, the only one thing such a Reality Marble could specialize in.