Another "dimensional" theory
by David KnottIt has previously been noted that both the Gold Box Immortals Set and "Wrath of the Immortals" make reference to "dimensions", but they use the terms in very different ways. To avoid confusion, I will refer to the dimensional references from the gold box rules as "dimensional orientation" and to the separate realities described in "Wrath of the Immortals" as "dimensional locations".
"Dimensional locations" are multiverses that are sufficiently removed from one another that Immortals cannot coexist in them (for mortals, "planes" serve pretty much the same purpose). "Wrath of the Immortals" describes a "Multiverse" dimension as well as dimensions of Myth, Nightmares, and Vortex.
It is the dimensional orientations that define how beings from different dimensional locations interact with one another, as moving from one dimensional location to another generally does not in and of itself alter one's dimensional orientation. In order for magic cast by one being to have its full effect on another being, the caster must occupy every dimension occupied by his or her target -- and even then the target has an enhanced chance to avoid the effects of a spell if he or she can perceive any dimension that the caster cannot. This creates a natural hierarchy of magical power as follows:
Mortals (the weakest possible spellcasters) exist in four dimensions but perceive only three.
Immortals exist in five dimensions but perceive only four.
Old Ones exist in six dimensions but perceive only five.
A natural effect of this hierarchy is that Immortals are completely immune to mortal magic and Old Ones are completely immune to the magic of both mortals and Immortals.
It is the factor of "dimensional orientation" that prevents some mortals from being able to affect other mortals with magic but makes some naturally poisonous to others. If we define the dimensions from the point of view of an inhabitant of the "Normal" or "Multiverse" dimension, then dimensions 1-3 are visible and the 4th dimension is one that is known to exist and is regarded as the source of magic. The "Myth" dimension shares the same orientation, so mortals from the "Normal" and "Myth" dimensions can fully affect each other with magic and also can touch each other without poisonous effects.
However, mortals from the "Nightmare" dimension have a very different orientation -- they exist in what "Normal" mortals would regard as dimensions 3-5 and regard the 2d dimension as the source of magic. Because they each exist in one dimension that the other does not, neither can affect the other with magic -- but each can poison the other with a touch.
Creatures native to the Astral plane create a slightly different situation. Regardless of origin, they perceive dimensions 2-4, which means that they share only two perceived dimensions with either "Normal" or "Night- mare" creatures. Their "hidden" dimension depends on their ultimate origin in the "Normal" or "Nightmare" dimensions. At best, a mortal spellcaster who does not originate from the Astral Plane will be able to perceive only two of the dimensions perceived by one who does. As a result, spells that allow a save for half damage result in no damage at all when cast by a "Normal" or "Nightmare" caster on an "Astral" creature or vice versa.
Immortals, of course, have a tremendous advantage over mortals because they perceive and exist in one more dimension than mortals. All Immortals exist in dimensions 1-5 and perceive any four of these dimensions -- 1-4 if they are of "Normal" origin or 2-5 if they are of "Nightmare" origin. They actually gain a bonus when using their magic against mortals of the same orientation, as they can perceive not only all of the dimensions that their targets can perceive but all of the dimensions in which they exist. "Normal" Immortals targeting "Nightmare" beings and vice versa face the same handicaps as "Normal" or "Nightmare" mortals targeting "Astral" mortals or vice versa.
Old Ones exist in dimensions 1-6 and can perceive
dimensions 1-5 -- which makes them completely omnipotent in their
dealings with lesser beings. While they would seemingly be unable
to perceive the 6th dimension, they obviously can use their powers
to manipulate beings from the Vortex dimension -- and there is no
question but that the orientation of the Vortex dimension includes
the "forbidden" 6th dimension. While "Normal" creatures would be
immune to spells cast by Vortex creatures, Vortex creatures use
natural "non-magical" abilities to subdue foes for their Old One
masters. Because they are so alien to "Normal" mortals, these
"non-magical" abilities are particularly effective.