The Abyss is the plane of abandon - nothing is restricted, no behavior encouraged or denied, all desires are dialed to their most wanton. The result: a grotesque melange of nightmares and mad dreams, sprung to life across hundreds of layers shaped around the whims of massive, unrestrained and tremendously vile egos. A realm of hedonism and terror that allows and expects the very worst out of its festering horde of natives, the Abyss is peopled by those desperate for the release it promises, the power to destroy and therefore unleash such desires as have been denied you. The lust for destruction and repudiation of order and structure manifest physically in the form of vast chasms that penetrate through the layers of the Abyss and the strange, inconsistent horrorscapes that exist deep therein.
Acheron is the plane of devaluation - all things are insignificant, valueless and incapable of escaping their place in the cosmic order. The peoples of Acheron live lives of terrible futility, participating in wars that will never end for honors that will never be accorded; struggling to survive for the right to live another day in a frozen and deadly hellscape; picking their way across a cosmic junkyard in the hope of finding some small scrap of worth before becoming part of the landscape themselves. This is not a plane without hope - rather, it is a plane where desire comes to die, a plane where the personal gives way to the faceless and meaning can only be found in holding some form of authority to differentiate oneself from others.
Arborea is the plane of inspiration - awash in passion, tranquility, history and festivity, Arborea's three layers are tremendously unlike one another, yet unified by the emotions they stir in travelers. The peoples of Arborea take life in both hands, desirous of joy, purpose and the freedom to marry the two. Below the familiar pastoral splendor of the uppermost layer lie the wild seas of Ossa and the myriad realms and cities that await in chasms below the shallows, followed by the mysterious sorrows of Pelion, land of white sands.
Arcadia is the plane of perfection - everything there needs to be right, because right means good. A militant plane by nature, particularly across the second layer, Arcadia is populated by peoples who have their own understanding of Right and are willing to put steel behind their definitions. Moreso than other Upper Planes, Arcadia's denizens are obsessed with purity, to the detriment of visitors who do not conform to the Arcadian notion of what is or is not desirable. As such, while Arcadia is largely placid and pastoral, it brooks no middle ground and is largely uninclusive. Its antithesis is Pandemonium.
Baator is the plane of coercion - all will be surrendered when the right button is pushed, even the self. A place of fantastical cruelty and subjugation, the peoples of Baator live out their existence fully cognizant of their place in the structure and the reasons they have been forced into it. The plane and its denizens thrive on the capitulation of others, the foundation of their twisted but absolute version of authority. Whether one desires it or not, a place does exist for all somewhere in the Nine Hells - typically under a devil's taloned heel. To get ahead is to understand the rules and how to apply them like the twisting of thumbscrews. To gain authority, power or prestige is to discover a different, more impressive boot atop one's back - for in the infernal hierarchy, no one escapes the crushing weight that is the will of the Lord Below.
The Beastlands is the plane of instinct - a place is there for all things that simply wish to live in tune with their natures, provided those natures are in balance with those of other beings. A plane largely bereft of anything that might be called "civilization," the Beastlands is the realm of the most primal exultation, and its peoples are one with the land and its true heirs, the animals. Those who try to oppose the essential nature of the plane will find themselves swept away, if they do not become prey first.
Bytopia is the plane of endeavor - a thing isn't worth doing unless it's worth working at. A plane of two natures, Bytopia is united around the ideal of personal commitment and effort, whether one is working for one's supper on Dothion or enduring the cold winds of Shurrock to hunt for game. Pride, not in oneself but in one's accomplishments and value to a greater community, characterizes the peoples of Bytopia. Welcome and shelter are available for any who are willing to earn or pay for it.
Carceri is the plane of contempt - the total distrust of and disregard for others and even the self. The peoples of Carceri are self-devouring, proponents of applying Tall Poppy Syndrome to every facet of their miserable, suspicious and paranoid lives. The other is denigrated and calumnized, for who would reside in Carceri except those who were cast out from elsewhere? Called the Red Prison for good reason, the very nature of the plane is one of harshness, isolation, division and futility.
Celestia is the plane of aspiration - there is a desire innate to the plane itself that those who come there seek self-improvement, enlightenment and a way to participate in the fullness of order and the greatness of good. The structure of the plane reinforces this concept, with the higher heavens serving as waystations on the road to self-actualization and absolution. The peoples of Celestia are strong believers in the power of good in all its forms as a force against evil and chaos, and are wholly convinced of the virtue of their plane and their moral stance.
Elysium is the plane of succor - help, hope and welcome are always available, even in the thickest fog and the darkest night. Its people are known for great joy and contentment, and indeed the plane is so pervasively winsome that it can be toxic and even fatal to those without the will to keep themselves separate from the temptations of paradise. While only the topmost layer is pastoral and inviting, each layer of Elysium is characterized by peoples of good will and staunch heart ready to serve as beacons against despair and bastions of empathy.
Gehenna is the plane of esurience - a terrible, grasping, gnawing need to claw one's way toward something approximating the top of the heap, at the expense of anyone who dares to have more than you. The peoples of Gehenna scrape out an existence on a plane with limited land mass available and all of it insistent on sloping toward an endless void, forever toppling those who think they've attained some measure of safety. Civilization exists because it has to - unity is strength, and numbers mean there will exist someone whose neck you can stand on to get ahead - but Gehenna is characterized by a pure and practically codified pair of understandings - that greed is all, and that the top of the pile is no safe place to be when your world is a volcano.
Hades is the plane of despair - nothing about the plane sparks even a mote of hope in those who dare travel there. Hades is pervasive, all-devouring hopelessness, three glooms filled with the toxic miasma of evil at its nadir. Pure. Terrible. Completely bare and unreserved. Evil on Hades is not dressed up or given flesh and ambition; it is capable of sucking the very life and will out of the people who enter the gloom, filling them with deadly sickness, or inspiring such tremendous emotions of grief and futility that its visitors and victims take their own lives rather than face the brutal and naked truth of this monolith of utter despair.
Limbo is the plane of idiosyncrasy - a totally unique experience not just of the self but for the self. The plane of Limbo is, by definition, what one makes it to be, subject to one's capacity to do so. It begs for ingenuity and the will to force a tiny piece of the multiverse to conform to your preferences, rather than the other way around. The peoples of Limbo are seemingly few in number, but all desire, or have found, some measure of personal freedom of a kind that simply cannot be experienced elsewhere. By its very nature Limbo demands the personal wherewithal to endure its essence and seize the day.
Mechanus is the plane of objective - there is a real, salient and true purpose for all things, or they do not matter. The plane of ultimate order has only one layer, for any possible permutations of the essence of Mechanus must surely be able to fit within the larger mold. The peoples of Mechanus tend toward lives of tremendous peace, for the necessity of function is ingrained in them and deviation is at odds with the way they desire to be. The strange beauty of Mechanus is a reflection of cosmic understandings that order need not mean straight lines and perfect symmetry; however, anyone who visits will certainly note the alien stability and immutability of Mechanus cultures.
The Outlands is the plane of choice - it is impossible to stand at the very center of balance, so by definition anywhere you go is closer to something. Unusual among all the planes, the infinity of the Outlands seems somehow compromised by the nature of its Hinterlands; however, there is still a radial infinity that accommodates the many deities who have chosen to establish their realms within the relative neutrality offered by the plane. The peoples of the Outlands run the gamut, each holding on due to some commitment to the principles of cosmic balance... or maybe just retaining the right to walk away and make a different decision. The Outlands has sixteen famous paths to the rest of the Outer Planes, but in its heartlands is the singular spoke around which the Great Wheel revolves.
Pandemonium is the plane of fear - there is nothing but the screaming terror of the winds and the dreadful claustrophobia of the tunnels, there in the darkness, waiting for all. A place of madness. Anxiety. Dread. Pandemonium's peoples have been eroded by the shrieking gale, abraded by the vicious winds and the insanity they portend to become something tough, lean and edgy. Strangely, Pandemonium does have communities - the bizarre esprit de camp that forms between the desperate and wind-scoured souls is real and salient, born up simply by the impossibility that anyone standing next to you could do worse than the nihilistic void of the tunnels. The plane itself may be screaming in terror, somehow perceiving the limits of its own infinities and the horrible gulf beyond reality; certainly the winds must issue from somewhere, and the plane seems to collapse inward on itself as one enters the lower reaches.
Ysgard is the plane of glory - the freedom to contribute the grandiosity of one's life and deeds to the greater song of the multiverse. An oftentimes challenging plane replete with struggle, challenge and battle, Ysgard and its peoples care less for outcomes and more for the deeds and stories of a life in the process of being lived. Death has little hold on Ysgard, making combat less a lethal endeavor to conquer and subjugate and more a method to test one's mettle and fire.
You'd already be in an area of particular darkness, so everything would get darker, then go eigengrau, then you'd feel as though your eyes had adjusted to the darkness and you were viewing part of the Prime in an area of limited light. Whether or not you'd notice the lack of color, well, that's on you.If so, how would you notice the change? Would everything suddenly turn black-and-white?
If you're going back the other way it's farcically easy to spot; rifts to the Prime look like massive glowy patches on the Plane of Shadow, a rather unlikely sight there.