Modelling Interplanar Magic

A fanart depiction of a Magic: The Gathering card.

A fanart depiction of a Magic: The Gathering card.

In order to refine the high fantasy magic system proposed for the Golarion GURPS campaign setting found herethis post addresses the issue of interplanar magic, or the idea of casting long-distance spells through the boundaries of dimensions. Multidimensional worlds and the existence of different planes are a common feature of high fantasy and in Dungeons & Dragons or Pathfinder, and this post seeks to gather current information from GURPS rules and to introduce new ones.

Planes

As Boundaries

[If the] laws of magic might also allow casting on targets in other dimensions or “realms” [it is with] difficulty. Casting across planar or dimensional “boundaries” is at -10 per boundary crossed, on top of any relevant -5 for being unable to touch or see the subject (p. B239). 1

It was mentioned in a small side-pane of GURPS Thaumatology, but the question of how magic interacts with planar boundaries was something that occurred to me and my players in our last GURPS game session. The author also takes note to ensure that these sort of penalties do not already affect cross-planar spells such as Gate spells or summoning.

The author also suggests that there is a chance that these sort of boundaries between planes can be weakened, or at the very least thinner in some areas where there is intermixing, intersection, or resonance between the planes. I can immediately think of an example of this in the Eberron world and associated planar orbits. The -10 penalty is halved for such situations.

Let us extend the ideas from the author of GURPS Thaumatology further. If two planes can be especially close, it could even be possible that two planes are especially far away from each other. In this case, we could extrapolate a penalty of -15, meaning interplanar magic between these two world should be extraordinarily difficult (almost impossible).

It is also suggested on M83 that the difficulty and cost of Gate spells may also differ with planes. In this regard, we can make a similar observation that passing through more planar barriers requires a -1 penalty to the skill for every boundary after the first adjacent one to reflect greater distances, and a potentially increased cost of 10% of the base cost for every barrier after the first (see Plane Shift and Plane Shift Other).

As Networks

The notion of movement between planes will be expanded upon here by considering this framework within the confines of a construct found in computer science.

The above suggests that if different planes are adjacent to each other and can be moved from one to another only from particular planes, one can visualize the relationship between planes as a graph where edges are directed between planes such that if one exists between a plane and another, its direction implies movement can exist between the source plane and the target plane and only in that direction. I spoke about this notion of considering graphs within the context of roleplaying games before.

Consider the following simple model created using GraphViz.

Two planes and two edges! Shocking!

A simple model of planes.

In this world, there are only two planes: the Material Plane, and the Bizarro Plane where bad things happen. We can even assign edges weights. In this case, the weights of the edges in both directions are equal, indicating that the no special behaviour is present regarding intermixing or resonance of the planes.

This is a graph of two nodes and two edges and can exactly represent the notion of planes with respect to the world we want to model. In here, magic that is cast in the Material Plane and only in that plane will acquire no penalty. If casting across the boundary to the Bizarro Plane, a -10 penalty will result.

But a model of planes can be as complex as one wants.

A (more) complex model of planes.

A (more) complex model of planes.

With respect to the world of Golarion, we could use this established set of concepts in order to try and model it properly.

Golarion

Determining the Model

In the Paizo supplement The Great Beyond: A Guide to the Multiverse, the hierarchy and mapping of planes is discussed for the world of Golarion. There is a suggestion that the Material Plane is a part of an Inner Sphere of planes where each plane straddles the next as concentric spheres. This Inner Sphere falls within a larger Outer Sphere with an incredibly large hollow centre, empty for all but the Inner Sphere and comprised by the Astral Plane. It is suggested that dead mortals reside in the Astral Plane, forming a large River of Soul, an obvious callback to another mythology.

Therefore, we can model this configuration of planes as follows.

A model of the Golarion arrangement of planes. (Source)

Consequences

Given this model, we can make a number of interesting observations.

  • The Ethereal Plane straddles both the Material Plane and its opposite, the Shadow Plane. Therefore, both are equidistant to each other but are separated by this plane in the middle.
  • The Elemental Planes have a very specific order; the Plane of Fire is furthest from the Material Plane and in order to get to it, one must go through three planar boundaries.
  • The Astral Plane exists outside the Elemental Planes, which is interesting since it is suggested that if extradimensional spaces are stacked in the world of Golarion (such as when placing bag of holding within another), that person is taken through a rip to this plane, suggesting a travel through multiple planar barriers.

Suggested Reading

Consider reading the following articles.

 

Notes:

  1. GURPS Thaumatology by Phil Masters
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