Reality pressure is an omnipresent fact of life for agents in their world. It is the anti-magic force, and would not exist without Fantasy. It is defined solely by the existence of magic and is a symptom of its existence in the world, a side-effect of the existence of agents and their use of magic. Sometimes referred to as the “will of the universe”, reality pressure is what makes magic difficult, fighting against it every single time it arises and doing its best to stomp it out. Because of reality pressure, gaining magical power and becoming stronger is a challenge, one that gets harder the stronger you get. In addition, any magical change forced onto the world will inevitably be reversed; space will inevitably snap back into proper form, time will inevitably resume, and physics will inevitably return to normal.
Exerted by Reality as a response to Fantasy, a single instance of reality pressure lasts only until the phenomena that birthed it is no longer active, growing more and more difficult to stave off the longer it has to stick around. While more capable agents can hold it off for longer through applications of sheer willpower or clever magic, it will eventually win out and reinstate Reality. Magical beings such as star spirits are not erased by reality pressure for reasons that are not completely understood, but it is thought that this is a result of ego pressure, the opposite of reality pressure.
Whereas reality pressure is the external force produced by the universe in response to magic, ego pressure is the internal force produced by an individual in response to magic. Much like reality pressure will eventually reinstate Reality, ego pressure will eventually overturn any change to the soul, healing injuries and reversing any attempts to manipulate its structure. Of course, ego pressure can be dealt with rather easily in most cases, by simply killing someone before it can kick in. And unlike reality pressure, ego pressure is powered by the individual and their own force of will, meaning that particularly stubborn individuals are especially hard to affect for longer periods of time.
Without reality and ego pressure, any given universe would descend into sheer chaos as the effects of magic make themselves apparent. Damage to the fabric of space and time caused by fights between high-level agents would make space travel impossible as missing bits of space would result in individuals overshooting by thousands of light years, if not more. Planets would slam into each other at faster-than-light speeds, killing billions as a side-effect of some moron bending space to dodge a barrage of attacks, or erasing space to avoid a long journey.
As horrible as this would be, however, things would get so much worse so much faster that it would never be relevant. The potential side-effects of even small scale magic are not to be underestimated. Magic is ultimately the manifestation of one’s own concepts and understanding of the world, and left unchecked, everyone’s competing realities would tear the universe apart. Reality pressure, however, forces these interpretations to remain in the soul and appear only for a short period of time when magic is used, while ego pressure maintains an individual’s identity and worldview in the face of this. Without either, life would be impossible, and aspects of reality would lose all meaning and consistency. Some impossible universes are situated so deep in Fantasy that this is how it is.