The Planar Membrane
The World of the Wilderlands excerpt below represents as much as any sage could hope to know about the boundary between worlds. In it, the authors refer to the material plane as the Prime. Their references to the Shadowlands refer primarily to the ether.
Excerpted from The World of the Wilderlands of High Fantasy:
This information would be common knowledge among adventurers.
The Prime is separated from the Outer Planes by the Planar Membrane. The membrane is a non-dimensional space at which every point of the Shadowlands touches on every point of the Prime. It is sometimes perceived as a gate, a pit, a door, a mirror, a cave mouth, a veil, a portal, a pool of water, or other place or thing that separates two locations or forms of existence. It is that which must be passed through or crossed to get between the different planes of reality, specifically, from the Prime to the Shadowlands, as the Shadowlands are the path that must be taken to all other Outer Planes. It is remarkably easy for mortals of the Prime to cross the membrane — all they must do is die, and their soul will awaken on the other side. It is rather more difficult for living mortals to cross over, and it is far more difficult for outer planar creatures to cross the Planar Membrane from the Shadowlands to the Prime without magical assistance on the Prime. Magical gates are the easiest way to cross over; their power often extends beyond the Membrane and through the Shadowlands direct to the desired Outer Plane. Natural gates exist in addition to those magically created, though they are rare and usually well guarded by men, spirits, or devils.
The membrane does not protect the material plane alone, but extends like a soapy film between any planes that are not coterminous. It is, however, particularly protective of some planes, most notably the material. Whenever an aperture connects two planes through any circumstance other than a complete planar breach, the Planar Membrane dampens or blocks any spell, power, or similar effect.
To cast an impeded spell, the caster must make a Spellcraft check (DC 20 + the level of the spell). If the check fails, the spell does not function but the points are still spent. If the check succeeds, the spell functions normally.
Specific planar boundaries may have additional (or stronger) effects:
- The material plane resists attempts for non-natives to enter it. For non-natives, any spells or powers used to cross the membrane are impeded. Spell-like abilities are similarly impeded, though in this case the user may take 20 on the Spellcraft check. This is how gith, drifters, janni, and other such beings ply the planes.
- Axis Mundi access points do not allow line of sight or effect from the destination side even while open, and do not allow line of effect from the Between to another plane.
- The boundary between the Ether and the material occasionally wears thin enough for mortals to accidentally wander through (treat as a complete planar breach), but under normal circumstances the Planar Membrane prevents spells and powers from reaching from the Ether to the material, and allows only abilities with the force descriptor to cross from the material. Ghosts and similar creatures who can manifest from the Ether become incorporeal in reference to the material, and can be targeted by spells and powers with a 50% miss chance.
- The Transdimensional Spell metamagic feat allows a caster not only to ignore the miss chance of targeting a manifesting ghost or similar creature, but it also allows the caster to target any other denizen of the Ether that it can perceive.
- The boundaries between any planes with opposed energy or alignment traits are particularly strong. The Membrane blocks spells and powers crossing between such planes, and impedes any attempts to use planeshift and similar effects to cross. Most travelers use an intermediary plane when crossing between such antagonistic planes.