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"The Phlogiston, The Rainbow Ocean, The Flow - an endless sea of every color" - gnome astronomer Hub-El, describing the unimaginably huge expanse between crystal shells
The phlogiston is that which is outside and between crystal shells. It appears as an infinite fog of swirling colors.
The Flow's vastness is dominated by flowrivers, which link crystal shells in a mind-bogglingly gigantic 3-D web. Flowrivers are useful to spacefarers because they (greatly) facilitate travel between the crystal shells. In many cases, a journey directly to a specific crystal shell would take many years, but the journey can easily be shortened to a much smaller amount of time by following the flowrivers from one shell to the next. (Hence, navigation in the phlogiston is a tricky connect-the-dots activity).
It is not possible to see a flowriver (though detailed measurements can detect a significant increase in the drift of the multi-colored fog in one or two directions). They are felt more than seen; an individual sitting on a spelljammer helm can easily realize that a starship has entered a flowriver.
Most flowrivers are two-way, allowing passage in either direction between two crystal shells. However, some are one-way, so it is not advisable to try to double back along a one-way flowriver (doing so poses the same difficulties as not using a flowriver at all).
Planar Layout Edit
The Manual of the Planes would have probably termed the phlogiston as a Transitive Plane had it been included in that volume, and in many ways, that's exactly what the Rainbow Ocean is. (It is still considered a part of the Material Plane, however, as are all known crystal shells). Nevertheless, there are features within the rainbow fog and even occasional phlogiston inhabitants.
Qualities of the Phlogiston Edit
The Flow is softly luminous, so light sources are not necessary on a ship's open decks while in the phlogiston. Visibility is quite limited, however - treat the phlogiston as heavy fog for that purpose.
Most places within the phlogiston look exactly alike, but there are known, and dreaded, Dark Regions from which few have returned. A feature unique to the Rainbow Ocean is the strange flow beacon, an object that resembles a rapidly spinning star. No one has ever determined who built these devices, nor what they do.
Fire in the Flow Edit
The phlogiston is extremely flammable within any kind of air, so the use of fire is highly inadvisable in the Flow. Even a spark can set off a small, fiery explosion. If a spell is cast that produces flame, it goes off instantly, centered on the caster, and it is considered to be cast as if on a plane with the minor fire-dominant trait.
For this reason, combat casters in the Rainbow Ocean tend toward the use of other energies besides fire, especially electrical.