When Nektos originally created undead creatures, he was planning on creating an army of powerful, sentient followers who would be in a forever deathless state. This did not go according to plan. He constructed a magical barrier that would isolate the soul from being reclaimed by the one mother, and created a magical virus that would keep the flesh of the creature "functional" despite being dead. Unfortunately for Nektos and arguably all sentient life, the soul still fled the dead, infected body and destroyed the original afterlife system in the process. The soul became the first ghost, unable to rejoin the one mother because of Nektos' barrier, and the body became the first of the mindless, infectious undead.
An adaptable god, Nektos began to worm his essence into his necrotic virus, and began to subconsiously control and guide the mindless undead that become infected with it. He usually does little more than keep them on "idle" however, building his armies for some unknown purpose in the future. An idling undead is still rather dangerous however, because it still needs to consume flesh to maintain its health and heal its wounds.
Corporeal undead can usually be destroyed by mundane means, but this is doing little more than dismembering the corpse. In many cases, leaving the dismembered corpse can be dangerous - given enough time it could reform itself, or be consumed by a scavenger animal that would then become infected itself. In order to truly eradicate the necrotic infection and destroy the undead on a more permanent basis, one must cast a purification spell. Fire can be effective in destroying the undead individual, but the virus in the ash is still contagious.
Husks - Husks are undead whose physical bodies are corpses mummified through natural means, either through freezing in high altitudes, chemical mummification such as in peat bogs, or dehydration in desert locations. Husks have leathery skin and a lurching gate that is limited by the stiffening of their joints and flesh. Despite its mummification, a husk's internal organs are very poorly preserved, and usually resemble soil or sand, depending on what is most common in their habitat. The actual infection that turns corpses into husks is located in the soil or ice of the husk's habitat, so there is little risk of becoming one regardless of how badly they injure you. (Unless, of course, they injure you to the point you die and your corpse is left alone with the infectious soil.)
Boneless - The best way to describe a boneless would be "an amoeba of flesh." Those infected with this breed of Nektos' curse quickly have their bones dissolve and become a sloppy, quivering mass. Boneless exist with a single-minded purpose - to envelop more food, to grow, and to find other boneless to merge with. Boneless are one of the more dangerous kinds of undead due to their resilience and ability to quickly heal injuries, and their ability to continue to move even once dismembered. Fire can be an effective deterrent, as can acid, when magic is not available.
Zombies - One of the most common forms of undead, a zombie is simply a rotting, decaying animated corpse. They have no special abilities, but their necrotic infection is highly contagious. Zombies have no natural resistance to rotting, so they are ravenous in their attempt to consume more flesh in order to rebuild their bodies. Any wound inflicted by a zombie's teeth, nails or bones can become infected, and without treatment the person infected will die and become a zombie themselves.
Shambler - Similar to a husk, shamblers are undead composed of graveyard soil. Like husks, the actual source of infection is the soil itself, and bodies that are buried in shambler-infested soil will become the seeds for new shamblers to grow. Shamblers are typically humanoid-shaped mounds of dirt, mixed with bone and other non-rotting material. Shamblers have two methods of existing - a dormant cycle where they are nearly indistinguishable from normal dirt, and an active cycle where the dirt pulls itself together into the shape of the corpse that seeded it. Shamblers are usually nocturnal, entering their dormant cycle during the day, and becoming their humanoid forms during the night. Because they are made of magically coalesced dirt, they are extremely difficult to fight without magical means - luckily their movement is quite slow and it is not very difficult to flee from them.
Creeping Vine - Undeath is not limited to only animal creatures. A creeping vine describes any kind of plant - even trees - who become infected with this particular kind of necrotic virus. As the plant dies, it loses any stiffness it once had, its leaves wither and die and any fragile components break off. What remains is a pliable tangle of "vines" that can move quickly, without warning, capturing and consuming animal life. Creeping vines are dangerous, and the infection is very easily spread. Kingdoms have been known to seed farmland with powdered creeping vine in times of war, and necromancers and wizards alike are known for keeping small samples of creeping vine in their labs, for use in spell components.
Fell Creatures - Created by a mutagenic necrotic virus, fell creatures die slowly as their body is taken over by the infection. Any kind of sharp pointed growth, such as teeth, horns, nails, claws and ribs grows into a double-layer, giving fell creatures a ferocious appearance, especially because in vertebrates, the second layer of ribs curves outwards, piercing the skin and turning into rows of barbs. Fell creatures can be any kind of animal, even godtouched - the infection is not discriminatory. The infection also spreads easily - any wound caused by a fell creature's natural-grown weapons can harbor infection. Fell creatures are perhaps the most insidious of infectious undead because the soul only leaves the body in the latest stages of infection - those who are beginning to show symptoms and growing the monstrous mutations may still be saved, even if they can spread the infection to others. In many areas, populations of fell animals are difficult to control, and fell pigeons are especially common.