This is an updated version of my Cuthun demon entry. It should be clearer what these beings are now.
Cuthun Demons (Also called Cuthunians)
Cuthun demons are in reality mortal beings which have all been changed by coming in contact with demonic essence. So they can all be very different depending on what kind of creature they were before they became a cuthunian demon.
They all share a few characteristics however, granted to them by the demonic essence that course through their bodies. This essence (also known as muk or corruption) makes a cuthunian demon able to survive things that normally would kill most living creatures such as surviving in a vacuum, without any form of nutriment, immunity to the ravishes of age and most poisons and diseases. Cuthunians, if not born adults only age until they reach that adult-hood and then stop.
They are also identified by that they are all magical creatures who can sense each other and are also detectable by silver who is one of the metals theyíre vulnerable against. Silver, mithril and dragon bone can all cause burns to cuthunians if in contact with their skin. These burns canít be healed by their normal regeneration and is also known for being able to pierce their various forms of magic as well. A person may protect themselves from the abilities of a cuthunian by wearing mithril however which is known to be capable of negate their effects, especially mental influencing ones.
Muk which is the life essence of these beings is a thick oily substance that shares many of the characteristics of blood. It's highly infectious to all biological life and have been known to in some specific cases even being able to change undead and synthetic life into cuthunians.
This change can just take a few seconds or weeks depending on the host which is then granted all the capabilities of a cuthunian in addition to whatever abilities the host had before turning into a Cuthunian (in some very rare cases a few abilites such as speech might be lost in the process). The exact nature of this mutation is not known but it seems to be tied to the personality and the abilities of the host. Itís known to be countered by dragon bone but other less known cures for it might also exist.
Types (or classes) of Cuthunian demons
The cuthunians divide themselves into various groups, Tuul (lesser), Reik (greater), Pau (pure), Soghv (true) and skaar (half-demons). Demons of a Ďhigherí status appear to be able to hold a form of mental influence of demons of a lesser one. This influence can take different forms depending on the class of demon.
Tuul or Lesser demons only defining characteristic is that they arenít sapient. These beings are usually under the direct control of a demon with a higher status than they are but are otherwise usually extremely aggressive, actively seeking out any non-cuthunian life to end it unless kept in check by an great intelligence.
Reik or greater demons are the various forms of sapient life which have been changed by the corruption. They tend to either life as free agents or work under the influence of demons with a higher class if they ever came in contact with them. Reik-class cuthunians appear to have a hard time resisting the commands of a pure or a true demon, but some have proven capable to.
Pau or pure demons are creatures which never have been anything other than cuthunians. They have been born directly from Cuthun the sea of muk all these demons spawn from and are said to be the clones of non-cuthunian creatures that came there long ago. Note that 'lesser demons' technically can be Pau classified as well.
Soghv or true demons are said to be the original beings which spawned from Cuthun and are the leaders of their kin. Soghv are characterized by the fact they are all immaterial and lack bodies of their own. Instead they use the great mental influence they have upon lesser classed demons to control and even possess them, assuming direct control. Some of them even build bodies for themselves.
Half-demons or Skaar only appear if a cuthunian breeds with another being (which may or may not be another cuthuinian.) These beings have unlike the other classes the need to sleep, eat and breathe and even age as their host race. They still share some of the cuthunians regeneration and immunities however but this is only true for the first skaar generation. All generations after the first lose more and more of what defines a cuthunian until they are just like their host race. Itís only then however they may be infected by muk again as skaar are actually immune to it.
Cuthun, death and gating
Cuthun, also known as 'nether space' by some is the birthplace of all pure and true demons (and many other types as well). Cuthun is at first glance a endless featureless plane of nothingness but in truth this world is completely filled with the demonic essence. Like dark sea of corruption it fills any being that enters it with it's essence, corrupting them completely. It is said that Cuthun in reality is a living being, or a 'dreaming god' as the cuthunians call it. It is said to be sleeping but have created all true demons from a dream which then have spread across the planes.
When a cuthtunian die its body quickly turns into pure energy which returns to Cuthun which will cause it to eventually to reform. This however is a very slow process (again, depending on the individual) and most cuthunians who do end up here usually are enslaved by the true demons and forced to fight in one of their many great armies. Any cuthunian can however if they so wish transport back to this plane by a method called 'gating'.
Gating is a method known by all sapient demons and is used to transport themselves between planes. This method is directly connected to cuthun as from here they may transport to any plane in existence. This is not a completely faultless method however, the one using this method have to picture the location he wishes to go. Unless he got a direct view of the plan (like on a TV screen or in a scry pool) the closest he can get is at a random nearby location near the place he wishes to go.
Gating also takes some time depending on the user; an experienced gater can only take about 1 minute or less to teleport while an inexperienced one can take days to reach his location.
A number of cuthunians specialize in this method of teleporation and can guide other cuthunians gating to certain locations which is often used to stage invasions from Cuthun.