Though Nektos created the phenomenon of undeath in the world of Aarn, the concept of "things-that-should-be-dead-that-were-still-living" was not a new one. Death channelers would often be corrupted by the essence of death flowing through their soul, cursed by their natural magical affinity to grow into twisted shells, walking corpses, living liches.
It was from this that Nektos gained the inspiration for his undead creations. He constructed a magical virus that could spread from infected to infected, killing them and then raising their body as an echo of a death channeler. Usually this would strip the corpse of its soul, but eventually Nektos perfected the process, and managed to create corporeal undead who still possessed their mortal soul such as vampires.
For a time, all corporeal undead fell into one of these two categories - those with souls and those without. Eventually, however, the wizards of man learned to harness the channeled energy of death that approximately one in every 240 humans had access to. These "lucky" humans became necromancers, death channelers who also learned the art of wizardry, mastering the art of animating dead flesh.
Necromantic corporeal undead are soulless, artificial, and have no infection to spread - so facing one in combat is slightly less dangerous than facing one of Nektos' children.
On the other side of the spectrum, there's noncorporeal undead, ghosts and spirits trapped in the inbetween through either magical means or sheer force of will. Ghosts are a type of undead that through means either magical or psychological have been overlooked by the current afterlife and are trapped between worlds. Ghosts quite usually go insane quickly - unable to dream, ghosts cannot heal what little sanity they have left by entering the astral sea.
Spirits, on the other hand, are the category of noncorporeal undead who still remain in this mortal coil through sheer force of will or by design of their soul. Despite not having a body, the force of their will allows them to remain in the inbetween, yet still travel to the astral sea in order to dream without being captured by the demons' afterlife system. Spirits are usually far more sane than ghosts, and may even be reasoned with. However, spirits are by no means guaranteed to be sane or rational.
All noncorporeal undead, both spirits and ghosts, have a number of methods they can use to contact the living. Less specific types of undead, and more forms of communication, these phenomenon can be used and harnessed by all noncorporeal undead. Noncorporeal undead are normally invisible to those in the material plane, and it takes great effort, energy, and practice for them to make themselves known.
Cold spots: Cold spots are generated when a ghost or spirit attempts to manifest, but lacks the power to appear visually, create an auditory manifestation, or to manipulate an object in the real world.
Smells: One of the first steps of manifestation, ghosts and spirits can generate unpleasant odors in order to attempt to try to ward off or attract living creatures.
Whispers: Ghosts or spirits who have concentrated more on communicating with the living manifest as ghostly sounds and cryptic, disjointed phrases.
Poltergeists: Mistakenly believed to be an actual species of undead creature, poltergeist activity occurs when a ghost or spirit has enough power to physically manipulate objects in the real world, but not create visual manifestations.
Possession: Rarer than other forms of manifestation, certain types of ghosts and spirits can possess the living. This can sometimes be an annoyance easily purged, a spiritual infection that lasts for years, or even a complete and total takeover, killing the host by shedding its soul, and then keeping the body intact and alive with the ghost or spirit's soul in place.
Orbs: The opposite of poltergeists, orbs are the product of ghosts or spirits powerful enough to create a visual manifestation, but not powerful enough to manipulate the real world. They are usually small points of light that gently drift through the area of activity.
Umbra: Umbra are proto-manifestations, stronger than orbs. They are black spots, shadows, vaguely humanoid or monstrous forms. A ghost or spirit that can produce an umbra will soon be able to manifest visually as it chooses, and only needs further practice. Once a spirit or ghost has learned to perfect an umbra, they have usually also learned how to create lesser forms of manifestations such as poltergeist activity or whispers.
Most spirits and ghosts who can fully manifest a visual form beyond an umbra can also physically attack living creatures, and communicate with them verbally, though there are exceptions.