The wiki explains it better than I can. The tl;dr version: Marble Phantasm is the ability of an Elemental or True Ancestor like Arucied to connect their will to nature. In Arcueid's case, she's been known to summon entire villages, create pocket dimensions where she has enough control over the atmosphere to create a vaccuum that can turn her enemies into puree, etc. She does this again in Melty Blood.

To be fair, I plan to almost never use this ability of hers, because of its game-breaky potential. If I ever change my mind and want to use it in a scenario, I'll be contacting the mods and players involved so I don't do anything godmode-y and step on any toes.