The inner world of a magical being which is manifested upon reality.
A Reality Marble is where the inner world of its user is materialized and projected upon reality. The effects of Reality Marbles differ between their users and have a specific set of rules and conditions that their user follows. A Reality Marble can't be gained by everyone, as only magi who could create it through years of research and training and magical beings such as demons, elementals and those among the Dead Apostle Ancestors are capable of gaining one.
Reality Marbles are technically not considered a part of normal reality since the inner world of its user is forcing itself upon reality. Because of this, the spirit of the Earth, Gaia, considers them an unnatural presence onto reality and regularly tries to force its influence to dissipate them. As a result, maintaining a Reality Marble requires a large amount of mana from its user and they can only maintain the Marble for short periods of time. However, the Marble of Nero Chaos is an exception to this dilemma since Lair of the Beast King is within his body and Gaia doesn't consider it unnatural as a result, thus Nero can maintain his Marble's effects for indefinite periods of time.