Hmmm... not really sure where to put this. I'd appreciate ideas for kingdoms and so on.
The two major planes in the setting are the mortal world and the underworld.
The underworld is divided into a number of intermittently warring nations inhabited by beings referred to as demons. Demons are not inherently evil and have a society similar to humans, though they are highly pragmatic.
Demons draw power from the suffering of mortals, though they can do this from their own world with no ill effects. A demon who enters the mortal world can feed directly on suffering he causes, but the longer a demon remains in the mortal world, the weaker it becomes, so that a demon can only gain any kind of benefit from this via mass murder (which is sure to draw attention). This degeneracy also has visual effects, most often cracked skin and glowing eyes - it is possible to guess at the length of time a demon has been in the mortal world from its appearance. As a result, few demons enter the mortal world who are not insane, and those are usually identified before they can do so. A summoning ritual counters the degenerative effects as long as it lasts, but can only be used to summon a specific demon. If the demon has increased in strength without the summoner's knowledge (see below), the circle may not be strong enough to contain it. Such a demon would be free to cause chaos without losing strength, at least until its summoning ends, and few would turn down such a delicious
All demons conform to one of a few dozen basic archetypes, with little difference in appearance between members of the same species. Under specific circumstances, a demon may transform into a member of a different caste through a process known as the Changing. This is not a linear progression, with some forms only attainable by certain other forms. Some demons who seek to assert their individuality or who regularly interact with mortals often adopt distinctive clothing or body modifications (It can be particuarly confusing when a brave merchant encounters a demon who has undergone the Changing since their last meeting).
Devils are a subset of demons who are exempt from this homogenity. Each devil possesses a unique (most often humanoid) appearance and abilities. While the strongest devils exceed the power of the strongest demon castes, the strength of individual devils varies wildly (it is thought to be related to age). Devils are also known to have stronger force of personality than demons, and often take up positions of leadership. At least some devils can remain in the human world indefinitely, but few have reason to do so.
Hellbeasts are the animals of the underworld - in truth, they are a class of demon themselves, though it is not clear if a sentient demon can be transformed into a hellbeast or vice versa. Many breeds are domesticated, though demons running nightmare stables are likely to belong to castes with fire resistance and so on.
At the centre of the underworld is a great chasm called the Abyss. Explorers who enter the Abyss have returned driven mad or not at all.
All undead are inherently evil, and thought to come from the Abyss - they are as much a danger to demons as to humans. Skeletons, zombies, and similar mindless undead are in fact constructs, the result of human spellcasters binding an undead spirit to an inanimate vessel to provide a source of power (it may be possible to use other types of spirit, but if there is a method it is not widely known). The bodies of these constructs can theoretically be constructed from any substance, though corpses seem to work best for reasons sages are not entirely sure about. Most true undead are incorporeal - corporeal undead are either extremely powerful or the result of an attempt to create a mindless servant gone wrong.
A demon transforms into a devil through a ritual requiring a human soul. He first removes a "soul-shaped" lump of the spiritual stuff which composes his body, then inserts the human soul in its place. This soul must be willingly given, and the newborn devil will sometimes reward his donor with knowledge and power.
Devils keep the existence of this ritual secret from other demons, claiming that they were chosen by the gods (souls have many uses, so a demon obtaining one would not draw suspicion). They also hide that the demonic soulstuff expelled by such rituals appears to be the source of all undead (humans may or may not contribute to their numbers in some way). Some devils have a "shadow", where the soulstuff managed to transform fully into an undead before losing cohesion and splitting up or mixing with that of others - such an undead is very powerful (usually being corporeal) and can recognise its former host on sight.
All undead are inherently evil, and thought to come from the Abyss. Mindless undead are the result of human spellcasters binding an intelligent spirit to an inanimate vessel to provide a source of power (similar to a golem). Undead bodies can theoretically be constructed from any substance, though corpses seem to work best for reasons sages are not entirely sure about. Most undead are incorporeal - corporeal undead are either extremely powerful, or the result of an attempt to create a mindless servant gone wrong.I'm not sure I'm following here. Are the undead basically like ghosts that are bound to a specific item? Or did you accidently switch corporeal and incorporeal?
I meant most natural undead are incorporeal - all constructed undead are the result of putting one in a body.
Can denizens of mortal world tell the difference of all those type of Demon/Devil?
Can denizens of mortal world tell the difference of all those type of Demon/Devil?Individuals are hard to tell apart, but the various castes look completely different (one could resemble a kobold, while another looks like a 20-foot tall octopus/wolf hybrid that spits fire). "Type of devil" is meaningless.
I was thinking of adding some Hindu/Buddhist elements to the demons, to tie in with the fact that they can change between different forms, some of which are little more than animals (even if becoming a hellbeast because of certain actions is impossible, the demonic public may not believe that).