13.9 The Astral Expanse

The true shape of the Astral Expanse is beyond a mortal mind’s ability to comprehend. It has empty fields of space interspersed with objects and terrain of widely varying sizes, ranging from small rocks to planets. Some of the terrain seems to be natural, while other structures seem to be carved with skill. However, everything there has strange, unnatural angles that boggle the mind. A collection of separate sculptures may all move together, revealing that they are somehow the same object. Small objects can have immense mass, and large objects can be strangely light. Tiny objects floating in midair that are almost too small to see can be sharp and immovable enough to cut anything that passes into them. Objects can fold in on themselves, disappearing entirely, or rapidly grow out of nowhere. There is no safe place in the Astral Expanse for a three-dimensional creature, even ignoring the threat of intentional attack.

There are four-dimensional creatures native to the Astral Expanse, but they are horrifying and utterly incomprehensible. They are called the precursors, because they predate the three-dimensional planes that humanoids inhabit. Precursors drift or fly around the Astral Expanse hunting for food, making travel through the Astral Expanse extremely dangerous.

As four-dimensional creatures, the precursors are essentially invincible in the Astral Expanse. Fighting a precursor is like the drawings on a piece of paper trying to do battle against an artist holding a pencil. At most, the drawings could only ever interact with the tip of the pencil touching the paper. Even if they destroyed the tip, they could never understand or harm the artist themselves. Meanwhile, the pencil can freely draw inside the drawings without "passing through" their outer edge. No matter how reinforced the drawing’s armor might be, it is irrelevant since the pencil could tap directly on their eyes or heart. Deities are sufficiently strong and uniform in composition to survive such internal attacks, but even they can only survive, not effectively counterattack.

The precursors are unable to directly enter or interact with the Material and Spiritual Planes. The flattening effect of the planes is lethal to their four-dimensional bodies, just like compressing a person into two dimensions would be lethal. As a result, they generally give the planes a wide berth. Nonliving objects from the Astral Expanse occasionally collide with the Material Plane. Typically, they appear in empty space as bizarre and misshapen asteroids, and they are either never seen or they burn up upon entering a planet’s atmosphere, making this phenomenon essentially unnoticeable. On extremely rare occasions, they may arrive close to a planet’s surface, creating inexplicable eldritch ruins on the ground or in caves. These become a hotbed of precursor cult activity if they are ever discovered.

Aberrations are the closest things to precursors on the Material Plane. They arrived fully formed on Nexus within the past thousand years, and their bodies seem to operate fundamentally differently than other life. Some fear that aberrations are an attempt by precursors to invade the Material Plane, though how this would be accomplished on a larger scale remains unclear.

The Astral Expanse holds ancient monstrosities of cosmic size and power. Fortunately, they live far away from the Material and Spiritual Planes. In the same way that deities have difficulty leaving their Spiritual Plane, the cosmic force of affinity makes it difficult for precursor leviathans to leave their distant homes. Instead, they reside in a distant region of the Astral Expanse called the Eternal Void. They can still reach out through intermediaries and cultists, as deities do. On rare occasions, a precursor leviathan may try to more directly interfere with the Planes. That would be an interplanar crisis which even deities fear.