Movement on the Transitive Planes
Characters generally must use spells or spell-like abilities to access a TransitivePlane. The ethereal jaunt and etherealness spells take you to the Ethereal Plane, the astral projection spell takes you to the Astral Plane, and the shadow walk spell takes you to the Plane of Shadow. Such spells should function in any location coexistent with or coterminous to the plane. If the Plane of Shadow and a particular Outer Plane connect, then the caster should able to cast the shadow walk spell from that Outer Plane. But if the two planes aren’t coexistent or coterminous, the spell won’t work. Matters become more complex when you cast one of these spells while on a Transitive Plane. Will a particular spell allow movement from one Transitive Plane to another?
Here are the answers for the D&D cosmology.
From the Ethereal Plane to the Astral Plane: You can move to the Astral Plane from almost everywhere on the Ethereal Plane, because the Astral Plane connects to everywhere on the Ethereal Plane. (The Ethereal Plane does not necessarily connect to everywhere on the Astral Plane, however, see below.) If you’re using an astral projection spell, your physical body remains in the Ethereal Plane, and the astral form moves to the Astral Plane. If you physically travel to the Astral Plane, your form loses its insubstantial nature and becomes solid.
From the Astral Plane to the Ethereal Plane: It is possible to move from the Astral Plane to the Ethereal Plane in regions where the two planes are coexistent or coterminous. If you are in a region that would not normally access the Ethereal Plane, no such travel is possible. A physical body moving from the Astral Plane to the Ethereal Plane becomes insubstantial (as the ethereal jaunt spell), whether it reached the Ethereal Plane by means of a spell or by moving through a color pool. An astral form can enter the Ethereal Plane, and it gains theethereal properties while there (it can be affected by force attacks or gaze abilities, for example). But if your astral form travels to the Ethereal Plane, that’s as far as you can go; you cannot manifest on the Material Plane. If you are forced onto the Material Plane, your astral form is destroyed and your soul returns to its original body. If you physically move from the Astral Plane to the Ethereal Plane, you can continue your travels by manifesting on the Material Plane at any time afterward.
From the Plane of Shadow to the Astral Plane: From the Plane of Shadow, you can use the astral projection spell, leaving your body behind on that plane (not necessarily a good idea, because the Plane of Shadow is the most hazardous of the Transitive Planes). You can physically move into the Astral Plane from the Plane of Shadow as well, because the Astral Plane and the Plane of Shadow are coexistent.
From the Astral Plane to the Plane of Shadow: There are no shadows per se on the Astral Plane (a gentle glow suffuses the entire plane), but the plane has locations of sufficient darkness to allow access to the Plane of Shadow. It is possible to move into the Plane of Shadow from the Astral Plane either physically or in astral form. A color pool is the most common way to reach the Plane of Shadow. When you use a color pool, your silver cord runs from the Plane of Shadow to the Astral Plane, then back to your original body.
From the Plane of Shadow to the Ethereal Plane, or Vice Versa: In the D&D cosmology, these two planes are coexistent with the Material Plane, but not with each other. Spells and effects that use the Plane of Shadow do not function on the Ethereal Plane, and spells and spelllike effects that use the Ethereal Plane don’t work on thePlane of Shadow. Travelers can’t go directly from one plane to another. If you decide that these two planes are coexistent in your campaign, then spells from one plane should function on the other. This means that your characters can have ethereal encounters on the Plane of Shadow and shadow encounters on the Ethereal Plane. Individuals traveling from one plane to the other would gain or lose their insubstantial ethereal bodies.