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.

For comparison purpose, following a similar metaphore, a Reality Marble would be, on the other hand, the ability to turn all the marbles into white ones; this is something the world wouldn't naturally allow. While this can be considered to be more efficient, this is, however, the only one effect such a Reality Marble could specialize in.