Sandestines and the Planes
A sandestine is a creature or being deriving from a different plane.
Or at least, that’s the simplest definition. There are exceptions of course.
‘Deriving from’ is a key point. Just because a being has come from a different plane, doesn’t mean they are a sandestine – even if they normally live there. For example, the races of the githyanki and githzerai now exist primarily on other planes. But they are not beings of those planes – having been, in very ancient prehistory, a people of the Prime Plane.
Conversely, the jann, although at home on the Prime Plane more than anywhere – being made of all four elements, nevertheless are ‘elemental’ beings, and thus sandestines.
Other exceptions include the tritons, another race of elemental beings, apparently originally from the Elemental Plane of Water. But, for whatever reason, they long ago left, and now exist mainly on the Prime Plane.
The important issue is that sandestines are not ‘natural’ beings – they are living manifestations and personifications of aspects of their plane of origin. So even if that Plane is in fact the Prime Plane, they will be a manifestation of some aspect of it.
Because the Prime Plane is the nexus of all other planes in existence, all other Planes are ‘images’ or ‘reflections’ of it in some sense (not a literal mirror-image sense, ‘shadow’ or ‘impression’ are also valid analogies). Different planes ‘reflect’ the Prime in different ways: the Celestial Planes, for example, tend to reflect the pleasant, noble and moral aspects, while the Infernal Planes reflect the painful, corrupt and evil aspects. The Plane of Fire, obviously, is a ‘reflection’ of all things hot burning, aflame, etc.
All Planes have at least some inhabitants, because they reflect the Prime, which has living, sentient, procreating inhabitants. Some of these inhabitants (the sandestines) are fairly like mortals, in form, abilities and even tastes – although all are somewhat distorted or idealised in the manner of their home Plane’s ‘reflection’ of the Prime. Most Planes also have analogues to many Prime animals and plants, which although sandestinal may not be very similar to higher beings of the Prime.
Sandestines are, in a sense, ‘incomplete’ or ‘out of balance’ beings, since they are only partial or lopsided ‘reflections’ or ‘shadows’ of true living things. Perhaps because of this, they tend (with some exceptions) to have great interest and fascination for Prime beings, and the Prime Plane in general. The nature of this ‘interest’ varies as much as the sandestines themselves – from love and responsibility, through curiosity and lust, to malice and even hunger…
Inner and Outer Planes
Demonologists and cosmologists maintain that there are three main kinds of planes. Firstly, there are the ‘close’ or ‘inner’ planes. These reflect the Prime most strongly in physical ways. Most important of these are the four ‘Elemental’ Planes (Fire, Water, Earth and Air). Some say there are other Inner Planes – of Energy (perhaps of both Positive Energy and of ‘Negative’ Energy (which some call ‘Void’ – the Plane of Absence), and some a Plane of Shadow (which is a Plane of ‘virtuality’ and ‘half-existence’). Some suggest a Plane of Light too (perhaps polar to the Shadow), which others contend is merely the Plane of Energy. There is inconclusive evidence for any of these hypotheses.
The second kind of Plane (‘transitive plane’) is not quite the same as the others. These are different because they, to a large extent, although not fully, coexist with the Prime – in the same time and place. They are therefore, less ‘reflections’ of the Prime and more like ‘surfaces’ or ‘edges’. These include the Ethereal Plane, which joins and coexists with all the Inner Planes, including the Prime. This Plane exists, to some extent, as a matter of perception and awareness. A being ‘enters’ the Plane by transforming their physical body into an ethereal one. This interacts only very weakly (or even not at all) with the physical world – being barely perceptible and perceiving the Prime only weakly. And yet one can travel in ‘real time’ and geography through the Prime in this state - and since physical matter or hazards have minimal effect this can be very useful.
“The Ethereal Plane subtly deadens the senses; it feels like a warm fog is surrounding you. Sights and sounds are muted. Objects give off only faint smells, and touch sensations are dulled.”
The other such ‘surface’ plane is the Astral. Although this plane does coexist with the Prime, it is much harder to use it to travel to a specific place, because it is the Plane of space and time and without a firm grasp on these ‘location’ and ‘travel time’ become rather meaningless – making navigation difficult. But it is used because, since time and space are more fluid it is possible, ‘through’ the Astral, to travel to almost anywhere, any world, any Plane, in almost no time. If one knows what one is doing – it is also horribly easy to get lost...
“Conversely, the Astral Plane makes the senses feel sharper. I cannot actually see farther or hear better, but my senses feel more alive. Motions seem sharp and clear, sounds are full and deep, and scents are distinct and almost pungent. Even the sense of touch is affected, heightened slightly so that the weave of fabric and the presence of a layer of ink on a page is noticeable. The wide-open nature of the Astral Plane creates a tickling along the nervous system. It has the feeling of air after a storm; the astral proto-matter continually renews itself, sparking the senses anew. Overall, I feel as though my senses are reaching great distances into a vast emptiness.”
Although the traveller above found Astral Projection to be stimulating, others have noted that, because time only exists astrally to the extent that one wills it to, there is no sense of life, vibrancy or change (and equally no aging or decay).
The Outer Planes – “in a distant mirror”
The true importance of the Astral Plane is that ‘through’ it one can reach the ‘Outer Planes’. These Planes are distant reflections of the Prime in terms of physicality (they may closely resemble the Prime in some ways, and may differ hugely in others. All the Outer Planes are realms directly malleable to the Will. Mortals familiar with the Essence Flows of the Prime find the Outer Planes suffused with what feels like the Essence. And Essence of a kind it is, but “not as we know it” . 
Outer Planes, although weakly reflecting the Prime physically, very strongly reflect fragments of the emotional and moral nature of the Prime. Because some people are kind and caring, there are planes where kindness and care are built into even the rocks and rivers, and whose inhabitants’ whole existence is geared to such emotions. Similarly, because some people are cruel, corrupt and malicious there are places where malice and corruption are found in even inanimate things, and the inhabitants whole being is one of cruelty and evil.
But this tie to the Prime also means that the inhabitants of the Outer Planes are deeply interested in the seemingly trivial affairs of mortals. Whether good or evil (or simply amoral), the outer sandestines are fascinated by mortal lives, mortal bodies and mortal spirits.
Since the bodies of Prime beings do not live for long in the world, many of the Outer Planar beings seek to persuade spirits of the deceased (and sometimes of the still alive!) to transition to the Outer Planes, at least for a season. The sandestines, it appears, crave the company of mortal spirits. Of course, since time may flow differently, many mortal sojourners do not realise the ages that may elapse in their time in the Outer Planes.
The realms of ‘good’ (collectively the ‘Celestial Planes’) are, generally, places of bliss and contentment – and it appears that mortals often don’t care to hurry their departure, and the sandestines there encourage good-hearted souls to stay. The ‘evil’ (Infernal or just Lower) planes of course, are usually, in one sense or another, prisons, from which mortal spirits stand little chance of escape.
Note that mortal spirits come to dwell in the Infernal Planes for a variety of reasons, including forced capture and foolish bargains with evil beings. But the most likely candidates are those homeless spirits refused entry to anywhere else (such as a Celestial Plane) and unable, for whatever reason, to achieve reincarnation, or collectivisation on the Prime. Most, therefore, tend to be outlaws and outcasts in life, who have so offended most people and gods alike that they have no defence against the soul-stealers. Since most such people have been evil and malicious in life there is a poetic justice to their fate that is not lost even on the Infernal Rulers!
Some important Sandestines
† these are beings native (adopted or natural) to the Prime, but are still elementals and therefore sandestines
Planes Beyond Planes
Different Outer planes have different relationships to other planes. Many link to the Astral Plane (and thus can be reached by gates and summoning magic from any other plane connected to the Astral Plane, not least the Prime). But many do not. To reach such planes beyond planes a different ‘intermediary’ plane must be travelled or communicated through. In some cases this is a mere technicality (although an important one to the spell caster!) as ‘gateway’ planes exist that link to further ones. The gateway plane of the Abyss is such a plane, known as the Plane of Infinite Portals. From it, spin off countless other planes (at least innumerable if not actually infinite) – collectively known as ‘The Abyss’. Because no demon prince rules the gateway, it is (relatively!) simple to magically establish a ‘relay’ point (for gates or communication) using the Plane of Infinite Portals that enables reaching almost any of the countless Abyssal planes.
Conversely, the gateway to the Nine Hells, known to scholars as ‘Avernus’ (the First Hell), is a highly controlled gateway plane. Access to the deeper planes (the other 8 Hells) is subject to tight procedures and authorisations – the approval of a senior devil is required to establish contact with the deeper reaches of Hell.
Although the planes collectively known as ‘Carceri’ (aka ‘The Red Prison’) are certainly not subject to the sort of organised rules as the Hells, the dominant force there (the demodands) ferociously guard access into (and most importantly out of…) the successively dark planes they control. The gateway plane to the others, Orthrys, is thus as far as most beings ever get by way of contact with Carceri.
Another group of planes is barely accessible from the Astral, except that it is linked by the trans-planar “River of Woe” (the River Acheron). Because no one being or group controls the River, and the River itself touches the Astral, it can be used to reach or communicate with any of the (at least 4) planes it touches, whether they touch the Astral directly or not.
The 7 planes of Skyrealm (sometimes known as ‘the Seven Heavens’), being another ordered place of rules, are organised similarly to the Nine Hells, with the gateway plane (Lunia, Moonrealm) being accessible via the Astral, and access to other planes being limited to those with appropriate rights.
Conversely, the various ‘chaotic’ celestial planes have a plethora of different and sometimes varying links and connections to each others and to the Astral planes. Usually, aid of a greater being of these planes is enough to find the ‘hidden ways’ (and since these routes change often, books describing them are often out of date!).
 Although the word appears to be a Perranic pronunciation of the Ancient Suel for “compact of destiny” (or possibly “what is set-down by law”) - Modern Aerdy sän destän “sacred destiny”, Mordenkainen asserted that it actually came from an ancient Baklunish root meaning “outsider” and this version is what is usually taught today to Brownrobes.