The Otz Chaim in Pathfinder
The planes of Pathfinder's Inner/Outer Sphere cosmology are already nested, so the linear structure of the sephiroth works very well as a ladder among them. Here we discard the more traditional mythology of the sephiroth and arrange their natures to suit the planes as described. Comments, as always, welcomed -- especially you, Shemeska! I'd love to get your reaction to this.
The Tree is not always a physical, living tree in Pathfinder. Some of the sephiroth take unusual forms indeed. However, since its form in the material world is a living tree, and it keeps something of this shape on the other planes, it is called the Tree of Life.
1.) Malkuth: Malkuth actually lies below the physical world. The roots of the Tree branch out between the Positive and Negative Energy Planes, one of the few known routes that passes between these two poles of existence. The sephira thunders with an incomprehensibly vast current, an infinite sea of positive energy cascading into a bottomless pit of consumption. The potential difference between these two mighty principles is said to be the driving energy that sustains the growth and life of the Tree. The sephira itself is a set of two pockets, one on each plane, each bracketed by permanent portals to the other plane of the pair and to the Material above. Between the two portals waver fractal interference patterns -- arcs of blinding white lightning sputtering into Mandelbrot fringes on the Negative, inky Moire patterns of shifting wavelets on the Positive. Each is reminiscent in shape of tree roots. The influence of the opposition plane in this region serves to dull each plane's usual damaging effects, but the current is so strong that someone without sturdy magic for moving or holding still will quickly find themself swept into the Negative and thrust out of the flow, there to suffer the full effects of exposure to the Plane.
2.) Yesod: A more normal tree, one of unknown species, sits within the Ethereal mists connecting the Material and Shadow planes. It is enormous, possessing its own gravity pulling toward a trunk dozens of miles around, its branches above disappearing into the mists and its roots reaching down to the fundamental energies of reality. It seems to wander through the mists, manifesting at this and that point in the various planes of the worlds. Those who wish to use the Tree to travel the planes generally seek the legends that tell where and when it will appear on Golarion, though the same books say it actually appears more often upon Castrovel. At astrologically significant moments, there appears the great Tree, usually in a spot on Golarion very close to nature, and to the shadow world, a dark and primitive wild far from the haunts of men. There a traveler can step onto the Tree's strong roots, and when the mists close around the Tree again, he is within the Ethereal and can advance upward. When the mists become dark and wrap thick around the Tree, the Shadow Plane draws near.
3.) Hod: As a traveler moves up from Yesod, the mists thin and he finds himself on the Plane of Air. Here the branches spread widest, and he journeys through a city-sized disc of wood and leaf, miles around and enthusiastically populated by creatures grateful for a solid roost. Some are djinn, seeing to the welfare of their plane; some are simply flying creatures seeking a nesting place; many are stranger things entirely. There is no gravity here; a traveller can bound from branch to branch up the Tree, if this is not his final destination. The Tree spins slowly and moves throughout the Plane, seemingly at the mercy of the winds, though it never collides with anything dangerous, nor does it ever seem to fall very close to other inhabited areas.
4.) Netzach: On the Plane of Water the Tree changes its form, becoming a great skein of kelp from which long fronds emerge. It snakes slowly through the plane, wafting in the murk.
5.) Tiphareth: The Tree is even stranger on the Plane of Earth: it is fossilized, and yet lives! While it seems to be nothing more than a vast pillar of rock in a giant cavern, imprinted with the vanished pattern of a long-ago living thing, it has been known to grow, and it heals when wounded. This does not happen very often -- the Tree's actual location on the Plane is a fiercely guarded secret of the native elementals, who under the command of their princes have organized a guard over the Tree's permanent and now well-fortified location. As has happened in several other places where the Tree has a steady presence, it has become an object of cult devotion, and thoroughly faithful humans and other species can also be found among the guards here. The shaitan, however, are unwelcome. Travelers are encouraged to move along to the next plane, or, if their business is on the Plane of Earth, to pony up the money for dedicated scrolls that will teleport them to their destination. Back as well -- but those scrolls are set to decay to uselessness if not used within a time limit, and are keyed to the buyers. Intractable planewalkers will simply be dispatched. Such is the harsh devotion of the beings of Earth.
6.) In the sephira of Geburah, the Tree appears once again to be wood, with exactly the result one imagines. It continually appears and reappears at innumerable different points in the Plane of Fire, instantly flashing in to flame and seemingly crumbling to ash, before reconstituting itself from its own ashes flaking away to vanish in the flames, revealing a moist green trunk that once again begins the cycle of consumption and rebirth. With each cycle, lasting bare minutes, the Tree appears at a new point in the Plane of Fire, making its use as a means of egress from that Plane a tricky business of timing and anticipation unless one is riding it through.
7.) In Chesed the Tree seems to physically emerge from the sphere which is the outer skin of the Plane of Fire, flames licking at its bace but failing to consume it. The traveler is now in the Astral, and the trunk begins to narrow. If he steps off here, he may find his own way to any of numerous demiplanes, or join the migrating river of souls to approach the domain of a god or one of the major Outer Planes. However, the Tree itself gives access to most of the major planes -- and since some predators are known to lurk about here with a taste for living flesh and spirit, perhaps the Tree is the safest route.
8.) The lowest of the final three sephira, Binah's branches begin to split more finely. The ends of these branches need no portals: they dip directly in to the Maelstrom, and into those cracks in the chaos which open up in to layers of the Abyss. Those who avoid these routes will arrive in a garden in Axis, where the Tree has submitted to have its appearing section pruned, trained, and sectioned so that watchers can spot traffic up and down the trunk. Travelers will if at all possible be detained and questioned as the the reasons for their travels; the axiomites and other citizens of Axis do not welcome disruptive influences from outside, including the living.
9.) Higher up, in the sephira of Chokmah, the Tree has ceased to be a green thing, and is a transparent concatenation of crystalline mathematical symbols, very much like the nature of the axiomites below, though recognizeable to those who meditate upon it as a different sort of soul-pattern. It seems designed to be as little disturbing as possible to the river of souls that passes through Pharasma's Boneyard, the main stop of this sephira. It is also immune to any sort of natural plague or rot in this form, which is vital as portals in the thin branches here also give access to the pestilent realm of Abaddon. The daemons are a strong presence in this sephira, swarming over it and seeking a way in to the Boneyard, from which Pharasma's will bars them. Those seeking Kether must deal with this final, fiendish presence before reaching the celestial realms.
(It will be noted that Hell does not have an apparent connection to the Tree. This is said to be deliberate on the part of Asmodeus, whose nine-layered domain is supposedly an inverted Tree of some sort itself. Could be pure screed, of course. What his intentions are with such a construct are, of course, as impenetrable as anything else he does.)
10.) In Kether reside three final branches strong enough to bear a traveler's weight. It is here that those who have triumphed past the elemental dangers, the suction of the Abyss, and the howling fury of the daemons come face to face with the lands of paradise: Heaven, Nirvana, and Elysium. The celestials will welcome them if their intentions are pure, but planewalkers will face a righteous host if their intentions are less than honorable.