The difference between necromantic undead and other kinds of undead creatures is largely a superficial one. Nektos has no dominion over necromantic undead, and instead of being controlled and powered by a special magical virus, the essences of necromantic undead are maintained through a combination of wizardry and death channeling. Necromantic undead come in many different flavors, and are created using well known and well researched magical recipes. Some of them have magical powers that rival true undead, but most get by on brute strength alone, and all are unable to turn other creatures into undead.
Necromancers usually can only directly control a limited number of undead at once, and so they often invest their money into enchanted items that allow their creations to behave autonomously. One such item is a "soul core" that has the ability to harness and enhance the strength of channeling effects. Another such item is a necromantic sphere, which is essentially a battery for channeled death magic. Because necromantic spheres give off so much death channeling energy, they often promote the creation of noncorporeal undead. Normally, a necromancer cannot create noncorporeal undead, even if they can influence and manipulate them, so necromantic spheres can be very handy, if dangerous artifacts to possess.
When a nercomancer dies, its undead pets continue to function, unlike the golems of an animancer. Nektos himself quietly usurps control over the abandoned creations - even though they don't have his virus, Nektos has become quite skilled at using death channeling magic himself, and can easily claim the necromantic undead left around after their creators' passing. This arguably makes killing a necromancer the most dangerous thing one can do when confronting one and his or her pets.
Animated Corpse - The equivalent of a zombie, animated corpses are one of the most basic forms of necromantic undead. Individual animated corpses are far more durable than zombies, partially to make up for the fact that they have no infection to spread. Animated corpses are usually abandoned by more powerful necromancers; they aren't quite strong enough to justify using up their precious channeling energy, nor are they valuable enough to justify using a soul core to make one autonomous.
Bound one - Only necromancers of quite some skill can create bound ones. A bound one is an animated corpse reconstructed from the ground up. Its raw material is a skeleton, overlayed with nothing but pure muscle and connective tissue. Bound ones are so physically powerful that they have metal bands encircling their limbs and torso. Without these reinforcing bands, they would literally tear themselves apart with the power of their physical blows. Bound ones are also quite agile, and very durable due to their metal reinforcements.
Composite Corpse - A composite corpse is a freak of nature - the limbs, heads, and natural weapons of many different creatures sewn together and magically bound. Composite corpses are a common "mid-level" necromantic undead - they can be created reliably by necromancers of average skill. Composite corpses are usually limited only by the imagination of the necromancer, and no two are exactly alike.
Demi-boneless - A cheap impersonation of an actual boneless, demi-boneless cannot grow in size or convert other corpses into boneless like their infectious cousins. A demi-boneless will remain the size it was at its original creation, which can range from a single corpse to a large pit full of them. They can still slither and digest creatures effectively however, and unlike traditional boneless, a demi-boneless can and will work in groups. As with boneless, a demi-boneless can be effectively dealt with using fire.
Mummies - One of the most advanced forms of necromantic undead, mummies are extremely powerful. Mummies require divine spells to create properly, so making one requires a priest as well as a necromancer. If a necromancer happens to double as a divine magic caster, he can make one by himself however. A mummy is created by meticulously preparing a corpse over a period of 70 days, casting divine spells along with necromantic ones in order to bind magical energy to it.
Mummies have a large number of abilities at their disposal not normally attributed to necromantic undead; They can drain energy out of living creatures in order to become more powerful, they can infect living creatures with horrible diseases that will kill them (but mercifully, not turn them into undead.) Mummies almost always are given a soul core which both allows them to act autonomously and also cast the divine magic they are infused with. Mummies are adept at casting curses and wraths, which can catch many undead-hunters off guard.
There are rumors of ancient mummies who, like liches, still have a soul, and thus, sentience. While it is possible that there are lost arcane arts that allow one to bind a soul to a dead body without Nektos' aid, it is far more likely that these reports came from those who mistook the presence of a soul core for a genuine soul. It is also possible that these reports were generated by those encountering an elderly death channeler who had simply wrapped him or herself in linens.
Projections: Even though necromancers who do not neglect their death channeling skills can communicate with, bargain with and influence the behavior of noncorporeal undead, many necromancers do not trust ghosts and spirits to be reliable servants. They usually are wracked with either pesky free will or annoying madness. Despite this, necromancers in general tend to find them useful, so it was only a matter of time before they discovered how to create a type of artificial noncorporeal undead.
By using a soul core, a necromancer can create a projection - an astral body without a soul. It acts as any other necromantic undead, obeying to the letter any commands its necromancer gives it. A projection shares its death channeling energy with its creator, and can use any channeling magic its necromancer has, though the two also share the same energy reserve. Beyond its ability to use death channeling magic, projections can also pass through walls, and turn invisible or intangible at will. They make excellent spies and assassins, though destroying one can be costly; the death of a projection has a small chance of shattering the valuable soul core used to spawn it.
Skelegons - Necromancers who are not skilled enough to create truly effective skeletons will usually go the "bigger is better" route. A skelegon is a giant skeleton, made from the bones of dozens of creatures sculpted into a humanoid shape. It's a popular - if sloppy - way to create a necromantic servant. Skelegons are similar to composite corpses in the fact that they can come in many shapes and sizes, and their natural weapons are normally limited by the imagination of their necromancer. Skelegons do not have the capacity to heal that skeletons do, however, their bulk makes them more difficult to damage in a more than superficial way.
Skeletons - Powerful, durable and frightening, skeletons are exclusively necromantic - they require a constant supply of death channeling energy in order to continue to function and move without tendons or muscles. A well constructed skeleton will be able to repair itself - even if its bones are shattered to dust - and continue to be an effective warrior or guardian despite the damage it absorbs. Many skeletons are not so well constructed however, and enough physical damage will cause the channeled energy maintaining them to dissipate. Magical damage on the other hand can be quite effective at disrupting their death channeled essence, and so magical attacks are recommended while facing them in combat.