Thanatos is the first of the Hellish Planes to exist, and still is the "biggest" and most populated one (the quotation marks is because this plane, like most, is infinite, so "large" or "small" is really just a comparison of the measure perceived from within by its inhabitants). This plane was originally created by Bhrater, one of three Übergods, the embodiment of light and consciousness, to serve as the prison for Three Rats, the Übergoddesses incarnation of destruction, corruption, evil and death. This impregnable prison should absorb the cosmic and divine essence of the evil creatures, but something went wrong; one of them managed to preserve her conscience, which in fact allowed her to become one with the plane. It is from then on that this plane was renamed Thanatos, fiendish word meaning "destruction", in reference to her divine soul. Of course, the other two rats were quickly removed from there and banished to elsewhere (the material plane, and some unknown location), to prevent the three eventually merging into a single quasi-omnipotent being. Unfortunately for Thanatos, as a deity, her new form did not last long: her state of existence was more than her soul could control, and began breaking out of control. From the very substance of the plane emerged new forms with their own life, evil metastasis trying to open a path in a wild eruption of life. Finally, Satan was the first to open the gap and tear the tissue of Thanatos, and thus of his mother, standing portentous amidst the immensity of the plane. After him, the whole plane convulsed in a long labor of centuries in which horrific creatures arose incessantly; even today there are still these occasional "births", with demons breaking through from the earth, an abhorrent antithesis of a burial.
The present appearance of Thanatos is that of a huge desert, actually endless. Where you look to, you only see miles and more miles of arid expanse. With the plane lacking any curvature, the horizon potentially extend to infinity, which merely enlarges the feeling of being lost in the desert in the middle of nowhere. Of course, nothing is further from the truth; one thing is that it's basically a desert, and quite another thing wold be to be empty. For starters, the morphology is not as uniform as it seems at first glance: in so much space, there are many geographical and terrain noteworthy features, although almost none of them disobey the norm of aridity and sterility that plagues this disagreeable plane.
Most of the plane is a rocky desert, whose floor is covered with gravel and dust; but there are also great sand deserts, or made of other materials such as sulfur. And, of course, the widely known "desert of bones" which is just that: a desert where the soil is composed of bones crushed up until they are nothing but a white powder, the bones of those who have perished at the hands of demons since the beginning of time, either violently or for making deals with them. In this desert of death some bones stand out, because despite being broken and chipped, are as big as trees; it is terrifying to think to what creatures they might have belonged, and terrifying to realize that, however great they were, they succumbed to the demonic hordes.
In addition to deserts, especially common in the valleys, you can also find huge chains of mountains, whose peaks rise beyond where the eye can reach, with the tops piercing the deep darkness which is the highest area of the mock atmosphere of Thanatos. Surely the dwarves would give anything to get to work in such mountains (and surely the demons would gladly take something from them, their lives). Some of the mountains are solid and dense as the most robust rock impregnated with ore; but others are pierced by endless caves and caverns. Thanatos has existed for more eons that mortals are able to imagine, and to calculate the number of demons the measure of "millions" falls short; but still not all the caves have been fully explored. Some are well known and frequented, because they act as a step to cross the mountains through the underground, a must for non-flying demons that would need months to surround or climb them; but many small branches and deviations are spread everywhere, reaching areas incredibly remote from the original entry. So easy is to get lost that demons try to avoid these less common routes for fear of wandering for eternity; the demonic folklore tells rumors and legends about demons that were lost forever, and let's not forget that these creatures are immortals, so in theory they are still looking for a way out...
Apart from mountains, volcanoes are a fairly common stamp of Thanatos. It's really hard to know where the magma comes from, as Thanatos has not a geological structure as that of a planet; it's as if the volcano were but a form of manifesting the anger and violence of the plane itself. These volcanoes remain in continuous eruptions, of unprecedented violence, and the roars are heard even before feeling the earth's tremors. The clouds of smoke are so great that some manage to get together with those from other distant volcanoes, plunging large areas of the plane in the perpetual darkness of an atmosphere saturated of hot ash, the heat of which is unquenchable. The rivers of lava run freely bulldozing everything, and the formations that arise once they cool are, at least, impressive and appalling in their magnitude.
But even though in the whole plane the heat is almost unbearable (apart from the fact that it has the very annoying peculiarity that it "beats", feeling puffs of heat to the rhythm of heart beat, to the rhythm of the simulacrum of life of Thanatos itself), there is also some solace in the scattered rivers and streams that run through the most unlikely places. These rivers may arise from any point, and flow without ever emptying into any sea or lake; they just submerge back underground to disappear and not continue. The route is not based on any logic neither, and they may even go up mountains; the only thing they all have in common is being extremely treacherous, because the bed is usually composed of sharp and pointy rocks, and rapids and eddies can appear in a seemingly random way, although the probability increases whenever something or someone is immersed in them. But the daring can enjoy a refreshing swim in their waters, or drink this fresh and crystal clear liquid; provided they don't find that the "water" burns as acid or turns out to be a deadly poison, something there's no way to know until testing is done.
And it is also possible for these rivers to get mixed with others formed from the rain of Thanatos, a rain that, as expected, is not exactly normal: gallons of blood are spilled on the surface, dying it red. Most deserts of Thanatos have a reddish tinge, which is due precisely to the remnants from the rained blood; remains that also emanate a significant stench of death unbearable for those not used to it. Demons, meanwhile, enjoy a lot these rains, and for them to bath in blood, or drink from the puddles that originate, induces them a state of frenzy and savagery greater than the usual in them. That's why the sound of rain is quickly joined by howls of rage and fury, and the sound of fighting, as soon as violence is unleashed.
To end this description of Thanatos, let's remember that this plane is inhabited, in addition to millions of demons, by their leaders, the Demon Princes. These monstrosities, led by the haughty and cruel Belzebuth, have a power that ridicules the mightiest of mortals from Ajsalium, and almost all the inhabitants of other planes; it is believed that only the gods exceeded them in power... and not all of them. These Princes of destruction have established their homes in various parts of Thanatos, always choosing those that best suit their tastes or characteristics. Whole armies, of slaves and servants fearful of the wrath of their masters, surround these buildings, that are also designed to withstand any attack, since amongdemons war is almost a routine, whether against the inhabitants of other planes, or between themselves.
Avernus is the Hellish Plane inhabited by devils, the second in existence. Properly speaking, the plane already existed long before the devils got to it, but then it had another name, Celestia, and other inhabitants: the angels. As it is, however difficult it may seem, Avernus was once a Heavenly Plane. But the "birth rate" inherent to the plane of Thanatos itself, coupled with the demonic promiscuity that gave rise to new generations of demons, soon created a foreseeable overpopulation. Of course, an infinite plane should be capable of hosting countless creatures, but these always have tendency to concentrate, and there's a reluctance to dispersion. So the demons decided it was time to expand their horizons, conquering some new plane on which to settle; this would later be continued with future conquests.
It was the plane inhabited by the angels the chosen one. No god lived in it, so it was quite unprotected in that regard; and although the angels have always been known as determined warriors, their small numbers did not seem enough to rival the invasion of demons. Of course angels were willing to make a formidable stand, and fought in a desperate resistance, sweeping away thousands and thousands of demons in continual battles and massacres. But numerical superiority was still overwhelming, and what is more, the demonic leader, Satan himself, was likely the best commander of the cosmos; the military campaign he planned was executed to perfection until the last angels were expelled from their natal plane and forced to roam the cosmos. This plane had a slightly lawful alignment, so that it were the demons withthe same alignment those who moved to it, plus some neutrals. The evil that brought with them eventually came to affect the plane; Thanatos, in turn, now holding only the demons of chaotic trend, "absorbed" this alignment. Regarding the demons that had migrated, over time, they started to being called devils, still bearing the name today.
The plane was renamed Avernus, who in the fiendish language means "abyss", due to it being mostly empty. Indeed, Avernus is basically a huge hollow dotted here and there with floating islands. These islands have a very variable size, and there are those which are only small rocks unable to accommodate more than a few buildings, up to giant rocks of irregular contour on whose surface have been growing continuously evolving cities, the largest of the which are the size of a kingdom from Ajsalium. Of course, not all of these islands are inhabited; most of those that are have an average size, enough to hold one or two normal cities; the biggest ones are reserved for the Archdevils, who establish their bases in them.
There's not just one single force of gravity in the plane, but each island has its own gravitational pull, which is more or less uniform regardless of the size of these; but there are exceptions to this rule, too, and some small islands have a gravity so strong that keeps everything in its surface flattened against it, while the gravity of huge conglomerates is so mild that with a good jump can one break away from it. But what all the islands have in common is that gravity only affects whom is standing in them; this means that, if someone were just a few meters from the surface, he would not be attracted to it even in a million years. In the rest of the plane, i.e. in the vacuum, there is no gravity at all, so flying through is free; but the lack of a consistent atmosphere hinders the normal flight of flapping wings, which finds little resistance enabling it to secure a big push forward. Of course the atmosphere is not so tenuous as to prevent breathing to creatures that need it (now, who in his right mind, other than a devil, might want to be there?).
As mentioned, among isle and isle there are huge expanses of emptiness, to which the devils often refer as "seas". It is not an inappropriate name, because despite the vacuum, this is more apparent than real, as there are ebbs and flows in it, and clear streams. These currents are very useful, as they allow communication with other islands, because the distance between them is often so great that without the help of the currents it could take years to reach even the closest one; and even though the devils are immortal, their patience is limited. As you might guess, the usual method of locomotion is flight, so the devils with this capability have a great advantage over their earthbound counterparts, which usually must settle for millennia without leaving a single island.
Of course, these also have their transportation. Besides the teleportation portals that exist between major population centers of Avernus, the vacuum seas are constantly traveled by a huge fleet of diabolical ships. These bizarre looking crafts are capable of maneuvering in three dimensions, something essential to navigate the plane, and are fueled with the souls of those who fell at the hands of devils; getting "fuel" thus becomes a perfect excuse for the continuous attacks of the inhabitants of Avernus to other planes, although, of course, it's nothing more than an excuse to justify ironically a taste for bloodshed and suffering. However, travel on board these ships is not without risks, as the vacuum, to be such, has a significant number of small rocks, barely the size of a shed, that float randomly or move on rapid processions. The impact of one of these rocks against a ship can destroy it, and any devil that can't fly will end floating forever... or until another rock directly hits it.
But there is also a curious element that seems out of place in Avernus: in one of the limits of the plane (although there is no way of knowing what limits Avernus in that area), lies a vast area of barren rock, thick and concave, far away from the nearest island for a whole eternity. Its size is so large that its concavity could hold an entire planet; not to mention the rather small islands of Avernus. Furthermore, from this remote desert the whole Avernus can be seen, in a strange vision that allows seeing everything, even what is behind other things; obviously, a vision so strange is incomprehensible to a mind that is not highly advanced, and may be uncomfortable or even painful. It's in this wilderness where Goleenban lives, the she-Archdevil of Solitude, and you cannot deny that there is no place more lonely than this in the whole plane. Only on the rare occasions when it's necessary to gather millions of devils in a single place do they move to these domains through portals that Goleenban opens.
Anyway, this is not the only danger of the vacuum, because although it often behaves like an ocean, other times it does as the sky. And when that happens, the storms that break are majestic. The clouds form from nothing, sometimes engulfing and trapping inside ships or islands complete with its citadels, and soaking all up. Literally, every biological being is at risk of drowning to death; and all, mortal or immortal, must endure an incredible strain on their bodies, numbing them. The deafening thunder rumble through the ether to immeasurable distances, and at any point of Avernus the sounds of thunder that clashed millennia ago can always be heard, still traveling. The lightning bolts light up the plane with a blinding light and a destructive power that could only be equaled by the wrath of the gods.
But the devils are already accustomed to these displays of vicious temperament of their plane, and are not afraid. They remain undaunted in their strongholds and cities full of proud and lofty towers, from whose watch points you can see the immensity of Avernus. Below activity is almost feverish, and the bustle of devils that go and come, attending the chores of Evil and their own, can be irritating to the ear. This noise is also joined by the constant shouts and cries of all those prisoners, hundreds of them everywhere, who constantly suffer torture at the hands of the very cruel and very efficient devils in the dark and damp dungeons that always spread, inexorably, like a spider web beneath the streets. And in the middle of the largest cities, in the top of their palaces, live the Archdevils, ruling with an iron hand and cruel law all their slightly loyal subjects; these semi-divine beings like to peek from their huge balconies, to watch their home and rejoice in the cries of their hosts.
Inferno, probably the word that wakes up more terror among most of the inhabitants of Ajsalium, among all those good people who only aim to pass through life with as little suffering as possible. Inferno is one of the three Hellish Planes, the smallest and most recent of them, but that directly affects the world of Ajsalium, which makes it the best known; to the extent that it is based on its name that all infernal creatures have been named. Inferno is a word from the common language of fiends (demons, devils and infernals), that means "doom" or "damnation": both for Ajsalium, that has to stand the continued aggression of its inhabitants, as for the souls of those who allow themselves to be corrupted, who after their death are sent to this plane, where almost certainly will be nothing but unfortunate creatures abused, enslaved and tortured by the most powerful infernals. Over the years, and due to the evolution of the common language, "Inferno" in many places has been substituted with the more generic "hell," a word present in all dialects of the world, used to refer to the worst imaginable things and with which to frighten children.
The origin of Inferno took place during the Long War, although it is difficult to establish the exact time, and there are several theories. Some feel that this plane was generated spontaneously by the Vortex itself, and it would be simultaneous with this; some delay it until shortly after, believing that it grew gradually; at the opposite end of the scale are those who think it was created by the Lords of Temptation and Inferno as a safe haven after their defeat in the Long War, or that it was Azeth who created it, before heading to his fate fighting Shratak Eternalflame, anticipating the eventual outcome of this duel. The fact is that no infernal, powerful or not, has ever revealed this knowledge to his heretical followers, either by ignorance, indifference, or perhaps because it may give a clue to their defeat. Anyway, Inferno is a half-plane partially coextensive with the material plane of Ajsalium, which means that some areas of Ajsalium, usually those within the deepest Underworld, gradually and subtly transforms into Inferno, almost without it being noticed.
Many priests, including those of the evil deities, like to tell horrendous descriptions of Inferno in their homilies, so to discourage all his parishioners to have dealings with infernals and abandon the true faith in the gods. And although many believe that these descriptions are nothing but an almost theatrical ploy to ensure the fidelity of the believers, and thus maintain their status and power, actually they are based on real facts that have been collected over the centuries from various sources, in an investigation that has been, at best, hard and painful like no other. Here we present the description of this damned plane.
Inferno consists of six "circles", although actually they are spheres (naturally). A magician expert in planar wisdom and fond of exactness (almost all of them are), would say that neither are they spheres, but are deformed and have an ovoid shape, since the Vortex (please recall that this is a great planar anomaly) exerts on these "circles" a remarkable attraction similar to the gravitational one. This attraction also causes that the circles are not concentric, but that the greater the "depth" the greater the attraction and proximity to the vortex, where really all these layers overlap tangentially. Of course, once in Inferno that does not really matter, because each circle or layer, technical name for the planar subdivision, is large enough so that its limits and form cannot be perceived. Just knowledge of the "off-center" which is the Vortex could be of interest, as a shortcut to reach any part of Inferno; although the wild energies released there prevent any living being from approaching without perishing, literally consumed by magical flames. We will now review the six layers from the outermost and most "friendly" to the innermost one.
First Circle: The first Circle of Hell has the peculiarity of not belonging entirely to this plane: it is both in Ajsalium as in Inferno. This circle actually is composed of all areas from Ajsalium that are partially coextensive with Inferno, producing a gradual transition from one to the other plane in this bastard layer. Getting to the First Circle is relatively simple: just enter the Underworld and choose the roads leading into the inner planet; sooner or later all creatures that an adventurous party may find would be infernals of one kind or another. If you do not want to travel the dangerous roads of the Underworld you can also opt for something as simple as digging; but the depths where the change occurs won't be reached with a small human excavation, but with a prodigious work of dwarf mining. In fact, this was the origin of hellish dwarves: they dug so deep in search for the prime ore used to forge black steel that they unknowingly reached the First Circle; its corrupting influence didn't take long to claim its toll on the bodies and minds of those unfortunate dwarves. These wretched dwarfs are one of the breeds most easily found at this Circle, and also a growing number of drow, both normal (as normal as a drow can be), and some hellish monsters created by Tylith, as the nathlice aberrations. Besides these, you can find many breeds of infernals, usually the less powerful ones (poor consolation). There is no Infernal Lord inhabiting this Circle, for it not being truly infernal; but that won't remove the possibility of encountering large patrols or even entire armies preparing a future invasion of the surface.
Second Circle: This Second Circle is really wholly part of Inferno, and any careful observer can find evidence to show that: for example, no mortal creature lives there except, perhaps, slaves that anyway will not live long. There is another very clear geologic evidence: while the First Circle is basically a labyrinthine set of caves in the rock mass of the planetary crust, reaching the Second Circle seems as if one is arriving to a new planetary surface, with mountains, valleys, plains, and a "sky" very dark but whose upper limit can't be seen. The change from the First to the Second Circle is rather abrupt and sudden, and it's usually like leaving a narrow passageway to get to a huge cave, so huge that it's the entire hellish layer. Otherwise, apart from the eternally overcast or nocturnal sky, it looks quite normal and even seems habitable. There are rivers, lakes or seas, which through the First Circle communicate with Ajsalium oceans. This is very well known to Dagon, one of the three Infernal Lords who have their abode established in this plane. Of course, it's the infernals who make this layer uninhabitable; they buff by the thousands or millions everywhere, and it is almost impossible to keep hidden from their eyes, for which the darkness of the Circle is no problem. In addition to Dagon, Belial and Uphir live in this Circle. Finding the home of Belial is no problem: it's just a matter of following the trail of infernals practicing lurid sex plays, and allegedly, miles away from his palace moans of pleasure and cries of pain can already be heard. Yet, the armies that protect each Lord, numerous as those from the greatest nations of Ajsalium, will give good account of intruders long before.
Third Circle: Neither does this Third Circle look too strange for an inhabitant of Ajsalium; despite being much less varied than the terrain of the Second Circle, this is similar to many areas Ajsalium: this is a huge forest in perpetual night. The way to reach this Circle is precisely to enter one of the forests of the Second, forest whose trees only have black leaves, and that the more we enter into the forest, that is, as we enter the Third Circle, become even more macabre and sinister. There is a superstition that says that this Circle was forged from the nightmares of Ajsalium residents, especially those of children; it may be true, because it brings together elements of the most gloomy fairy tales. The trunks of the trees are twisted, and its rough branches move to a non-existent wind, lashing anyone standing there; ghosts of dead animals appear and disappear continuously at the periphery of sight, and the rivers and streams are of stinking feces from which monstrous creatures drink. The wandering undead are numerous, as they form the bulk of the followers of Mania, "Queen of the Dead", one of the Infernal Ladies who dwell in this Circle; the others being Habondia, "Queen of the Fairies", surrounded by small twisted infernal fairies, as evil as seemingly harmless and helpful, and Nocticula, "Patroness of Witches", protected by thousands of cats in which reincarnate the souls of many of his most trusted and advanced disciples. These are not the only inhabitants, as herds of fultaurians, yrbesnaks and other beasts like to hunt in this infernal forest.
Fourth Circle: The Fourth Circle of Inferno no longer has any resemblance to any part of Ajsalium, except the most arid deserts of Rynath or Supterra. Basically it is a seemingly endless stretch of parched land, infertile and barren. Indeed, this Circle is quite similar to Thanatos, the native hellish plane of the demons, although this similarity is merely coincidental. It is reached by getting lost in some places of the Third Circle, where the twisted vegetation is scarce and the weather turns warm so obviously; after much wandering, the ground underfoot becomes a vast sea of sand and dust. Like any material desert, this is an extremely inhospitable place, with a high temperature, and an annoying soft light that seems to come from nowhere in particular, and that takes strange tones and shades of colour when it to passes through the mists and fogs that virtually fill the entire atmosphere of the layer. These mists are not wet, but actually it's made of the vapors that emanate from the many highly toxic chemicals that make up the dusty desert floor, as well as those that arise from some geyser-like wells. Throughout this Circle, there is nothing remotely similar to a plant and not a single drop of edible water, the only liquids are those of some scattered magma rivers; only the infernals can survive in such a place. Among these, we found two more Infernal Lords: Belphegor and Leonardo. The former enjoys the climate and morphology of the layer, and has built his palace amid a sea of sulfur; the latter has chosen this Circle because it's the one that offers him the greatest solitude and quietness, as befits his serene mood.
Fifth Circle: The way to reach this Circle is the most complicated of the top five layers, as it must be done by flying from the deserts of the Fourth. If sufficient height is reached, there will come a point when the sight of the desert and even its mists is lost, and in which one is in a apparently empty space, buffeted by winds of all kinds. This reminds a little to Avernus, but neither should this be given any more importance than mere chance. What is important, really, is that it is better that the ability to fly does not depend exclusively on purely physical means, such as simple wings, as the strong winds are the most typical characteristic of the layer, filling it in its entirety in brutal and chaotic ways: unbearably hot gusts of air currents collide with freezing drafts, forming tornadoes and whirlwinds of irresistible force. Around this Circle exist just a few zones of land areas, and even these come flying through the air occasionally to again stop "settling", apparently in the middle of nowhere, a few days later. This morphology makes the Fifth Circle the ideal place for all flying infernals, although the force of the winds keep away the weakest ones: Lords Eblis and Samael clearly do not belong to this group, so here they have their true home . They let themselves to get driven by gales across hundreds of miles, living without ever stepping on land, and accompanied by many followers who can't fight against the winds, nor would they try anyway, as they are cheerful to accompany their masters.
Sixth Circle: The sixth and final Circle of Hell is a terrible place, to say the least; terrible even for most infernal creatures, including some of the great Lords. This Circle is a huge void, an infinite well of darkness and evil beyond all physical or rational law; it's size may be as large as the infinite, and yet its walls constantly compress all who are contained therein; a single second becomes an eternity, and eons pass in less than a minute. Needless to say that no mortal creature, even the most powerful of dragons, could stand a stay in a place like this, where the flashes of magical energy that penetrate through the Vortex consume all matter with licks of unbearable pain; the Sixth Circle not only is the nearest to the Vortex, but supposedly identifies with it: it would then be the eye of the hurricane. Crossing the Vortex is beyond the reach of anyone less than a deity, and entering this Circle seems impossible for anyone not supposed to be in this heart of darkness, since there is no clear path or access to it from the rest of Inferno, nor teleportation portals. Those who must be there (the Infernal Lords called to Council) simply appear in the Sixth Circle as if having been absorbed by it; something that perhaps is more than a simple metaphor. That two Infernal Lords have chosen this Circle to spend their millennia of life allows us to form an idea of what degree of corruption they've achieved; the evil of Abaddon and Azrael is so pure that it is, in its way, perfect; the terror that they wake is, therefore, understandable. In addition to them, also the eight Lords of Temptation, supreme masters of Inferno, call this Circle their home; there they periodically meet in the so-called Infernal Council, located somewhere in the Circle that's, as expected, impossible to find for anyone who is not a member.