Two months ago I wrote about an idea of one day running a campaign in a downsized Planescape setting that has only 8 outer planes and 4 inner planes. Planescape has been a major influence on the Green Sun setting in general, and my ideas for the Spiritworld in particular. And so soon after writing that post, I went ahead to try out combining the two ideas for a campaign. While I had some good idea what I want the realms of the spirits to look like for a long time, I never actually got around to nailing them down into something tangible and specific. The planar system that resulted from toiling in the dark for many nights takes ideas and concepts from the planes of the Outlands, the Beastlands, Arborea, Pandemonium, Carceri, and Gehenna, and is tied together by a combination of the Plane of Shadow and the Ethereal Plane with several influences from the Gray Wastes of Hades. There are no dedicated elemental planes and no Astral Plane. Perhaps most curiously, there is no dedicated Material Plane either. As a result, the arrangement of the different realms is quite different from the way they work in Planescape.
Nature of the Realms
The world consists of an indeterminate number of realms that fall into two primary categories. The Spectral Realm and all the other realms. The other realms can be thought of as Corporeal Realms and people often divided them into Wild Realms and Underworld Realms, though that distinction is a subjective judgement and not based on specific distinguishing traits. The Wild Realms tend to be more similar to the Realm of Mortals, while the Underworld Realms are generally more inhospitable and their creatures more alien.
While the Corporeal Realms are generally separate from each other, all of them overlap with the Spectral Realm and are inseparetely tied to it. Mortal beings are native to the Corporeal Planes, while all spirits, which includes fey, elementals, and fiends, are native to the Spectral Realm. Mortal creatures can physically leave the Corporeal Realms and travel into the Spectral Realm. For spirits it is quite different. Spirits have the ability to project themselves into a Corporeal Realm without leaving the Spectral Realm. In fact, it is impossible for spirits to leave the Spectral Realm.
The Spectral Realm
The Spectral Realm mirrors all the Corporeal Realms, though there is a perpetual gloom and all colors are faded to almost gray, with the landscape appearing more like shadows than actual physical matter. It mirrors all the Corporeal Realms at the same time, resulting in a landscape as if someone had cut the maps of the Corporeal Realms into countless pieces and assembled them together into a single giant map at random. By travelling through the Spectral Realm one can reach any place in any Corporeal Realm. The difficult part is to find it. Fortunately for spectral travelers, the nature of time and distance seem to be very different in the Spectral Realm and if the right path is known seemingly every destination can be reached in just a few days.
However, keeping track of time in the Spectral Realm is difficult and its hard to determine how much time one has actually spend there. While staying in the Spectral Realm, mortal creatures are unable to fully fall asleep and gain no nourishment from food, making it impossible to take a long rest. They soon start to feel slightly tired and hungry but it never becomes unbearable, though for every day spend in the Spectral Realm they gain one level of exhaustion. If the exhaustion kills them, their physical forms fade away and they turn into shadows.
Spirits come in three types. Fey, elementals, and fiends. Celestials and intelligent plants have their creature type changed to fey, while aberrations have their creature type change to fiends. Generally speaking, fey are only encountered in Wild Realms and fiends only in Underworld Realms, as well as in areas of the Spectral Realms that correspond to them. But since there is no hard distinction between the two there are some of the Corporeal Realms where one might encounter representatives of both. Elementals are neither fey nor fiends and they can be encountered in all the Corporeal Realms and anywhere in the Spectral Realm.
The Wild Realms
The Wild Realms are Corporeal Realms with environments quite similar to that of the Mortal Realm. Most are dominated by forests, mountains, and oceans and are full of life in many forms, much of which appearing very familiar to mortals. For some reason the Mortal Realm is rarely visited by physical manifestions of spirits, which many scholars believe to be in some way connected to intelligent humanoid mortals being native to it. In the other Wild Realms, fey and elementals are much more frequent and the forces of nature appear to be even more powerful and unpredictable. People have told tales of realms where it is always night or where the sun never sets, where it is snowing all year or the mists never dissipate. While many of the Wild Realms are majestic to behold, all of them are considerably more dangerous than the wilderness of the Mortal Realm.
The Underworld Realms
Compared the the Wild Realms, the Underworld Realms tend to be much more barren and desolate. Many of them appear to exist entirely underground without any surface, which gives them their name, though there are also numerous realms that appear as barren hills or deserts. Most tend to be dark or gloomy, but again this is not a universal rule. Storms are just as common as in the Wild Realms, often driving before them clouds of dust or ash from constantly errupting volcanoes.
Borders between Realms
While the Corporeal Realms are generally separate from each other, they do occasionally touch and form border regions between them, through which creatures can travel from one realm to the other without going through the Spectral Realm. Border regions are not really in one realm or the other, and the environment blends traits of both of them. Often these regions are difficult to notice and the change in environment only becomes fully apparent once travellers have crossed fully into the other realm. Border regions generally have two edges that allow passage to the two realms they connect. Often these edges are found at the entrances of mountain passes, cave mouths, or at different points along a river that runs between two realms. But in many cases they just exist in completely unremarkable spots in the forest. Some known edges have been marked by either mortals or fey, which can take the form of lines of unusual trees, thickets of brambles, or carved posts made from wood or stone. Border regions also exist out at sea, but these are particularly difficult to locate and map.
While many border regions stay in place for a very long time, they are not entirely permanent. Some might in fact be quite short lived, but are never discovered or their locations shared among scholars and hunters. Other border regions are only accessible during specific times. These could be specific months of the year for example, or only during night at a full moon. There are stories of islands of the mainland coast that can be reached only for a single night every year, or ships disappearing without a trace along routes where no signs of a border had ever been noticed.