Good ideas and nice concept from BM on this.
When I think about Necromancy. No such thing exists, but in media there are various mechanics. Some deal with raising the dead that persist as undead over centuries or even indefinitely. Others deal with raising dead for a purpose (combat drones). A game, of course, needs to be specific, because it involves mechanics and such.
Just some theories.
1) I like the idea of a "necromant field" as a revival technique. In reality that would mean you stand in an area and draw a circle around yourself with the cursor (circle needs to be closed). Within this circle you can raise dead creatures, who all raise up aimlessly.
How long they persist, how far you can draw the field (during drawing, the circle dissolves if you draw the field larger than you have the power) and how effectively the dead are animated is not a choice, but depends on your individual power.
Reviving them means you give them a package of your power, you charge them up and they prevail for a peroiod of time nourishing from the power you gave them (which is life force). You can shut them down by taking the power away from them and they all fall to the ground dead.
To perform heretic necromancy (self sustaining undead indefinitely = curse) you need to perform a special arcane dark form of necromancy to give the undead the ability to draw life force from the environment (killing it slowly). The devastating thing about that form of necromancy is, that the undead seek life (to kill it and) to persist, all that without consciousness but through the "undead instinct". This form of necromancy can be used for mass genocide, but once the undead took over a town and kill every life, and are unable to travel further distances, they fall into hibernation (living dead lying dead on the ground still living, but being dead
). Interesting is that these undead do not decay further, but somehow prevail rotten as they are, for centuries.
When a traveler passes this dead town, he revives the undead close to him with his presence and they awake and the taveler is in deep shyt. Tricky thing is, that the corpses channel the force and radiate it to others, so the revived undead are able to reanimate other undead who are too far away from the traveler. Needless to say that nature avoids these places and animals don't go there and plants don't grow. Nature's way to isolate the cursed land.
There are rumors of such undead cities somewhere to the north. It is claimed that some powerful Thaumaturges possess the power to mask their life force siganture with advanced thaumaturgy making themselves invisible to the undead and that way traveled the ancient undead cities and gained invaluable ancient knowlegde.
2) To go back to the practical uses. The raised dead can be left alone to travel aimlessly. They will ignore everything as they contain the life force you gave them, but they will become notably hostile and try to kill things when their force package is close to depletion. This is the moment where they will become hard to control, untill you lose control of their actions once they go looking for brains to eat. You can gain control, or sustain it, by transfering more energy to them.
3) You can "program" them and give them tasks that they can perform by themselves, but you can also click on them with the cursor and move them the way you want (like a puppet). Because of the WASD movement system you can move the undead and follow them independantly.
4) Moving skelletons are not necromancy, but a different kind of evil. I don't know which, but I would think of spirits who wear bones as garment. Perhaps even their own remains from the times they were alive.
5) Killing undead? As I mentioned before, their rotten bodies are conserved unnaturally. If you tear off an arm from an undead and carry it in your backpack as a trophy, it will make you sick and tired, because it still draws life force from you.
An undead without a head still "lives", but it needs the mechanics of the head to be able to walk and keep balance. A separated head can still bite, so beware!!! A powerful necromant can still actively control a headless undead like a puppet master and make him stand up and walk.
Anyway, that is how I perceive necromancy and undead. I like there to be actual mechanics that are on the one hand understandable and realistic and on the other hand abstract and surealistic.