This deals mostly with clichés of fantasy, as usual, rather than individual things that authors have done or what necromancy was originally.

1) If you mean your necromancers to be scary, write them that way.

At this point, making necromancers into “typical” villains, just as with demons or vampires or werewolves or Dark Lords, doesn’t work. Black candles and dark rooms and chants and chalk circles aren’t enough to scare the reader; they’ve seen it all before. Neither is purple language like “eldritch depths of the stygian catafalque” going to make anyone cower. You have to give your audience a reason to feel like they’re reading horror or dark fantasy (whatever you choose to call this genre that flirts with both). You can’t just expect them to feel that way.
Some good ways to make necromancers scarier:

  • Use very straightforward, almost casual language, or an observer to whom a necromantic ritual is usual. This can make the scene seem surreal, uglier or more desperate in contrast.
  • Invent your own tropes and types for a necromantic ritual. So your mage wants to speak with the dead. Does it really matter if he confines them in a circle? So that’s what happens as usual in fantasy. Who cares? Is your fantasy world supposed to be connected to those worlds where it happens? No? Then you don’t have to imitate them. Perhaps the mage actually has to enter the “space” he creates to speak with the dead and wrestle them into submission before they’ll answer him. This might not be the usual necromantic thing to do, but it’s the trope in tales like “Tam Lin.” For every complaint that, “That’s not the way it happens,” you can find a different tale where it is the way it happens.
  • Invest the ritual with some risk. Yes, yes, gross things happen during a ritual, the evil necromancer sacrifices his young apprentice, the dead zombie eats a virgin. Whatever. What if the risk is mostly to the necromancer himself? If he has to bleed to feed the dead and drift into a trance-like state to hear their voices, is he going to come back from the trance-like state in time to prevent himself from bleeding to death, or the dead from draining him? Ethically risky magic could be more practically risky as well, to show how dangerous it is, and that necromancers are potentially mad- or have great courage.

2) Decide how much gore you’re going to show, and be consistent.

You could certainly have your necromancer conversing with rotting corpses. Few authors do. But it bothers me when an author does just one scene with a rotting corpse, to “give the feel” of necromancy, and has the rest of the necromancer’s informers or servants be svelte vampiresses or pairs of drifting eyes. (Another thing to remember: Necromancers can speak with the dead. It’s a form of fortune-telling in origin. Fantasy necromancers who don’t send armies of the dead marching across the world aren’t being stupid. They may not be able to do anything more, if the author is being true to that older tradition).
Necromancy has a stereotype in fantasy, but the details often differ from author to author, the way that the details of an elemental magic system will. I go into a fantasy book ready to be alert for what the author is telling me. What are the powers of this magical system? Who can perform this magic? Is it rare or common? What are its limitations? (Too many times, that last question is not answered, or answered only in inconsistent ways). I generally accept that the first time the author shows me something is a kind of truth-testing ground, and if details differ significantly after that, I wonder what happened. In particular, I dislike it when the author starts with an “emotional” scene, such as a gore-splattered one, and after that makes everything else more muted, as if once the reader’s attention is won it will always stay involved. Necromancy goes along with every other system of magic in this regard. If things are going to be markedly different later in the book, you will want to explain why.

3) The limits of control over the dead had better be explained.

If the necromancer can only command them to speak with him, and only at great risk, then no, I wouldn’t fault him for not doing more with them. But if the dead can be raised and used as servants, why not? (Having the heroes always too virtuous to do such a thing makes me vomit). If they can be raised and used as armies, why hasn’t someone done that?
If the dead lie, then that might be a limit on truth-telling. How do the necromancers have to adapt to ferret out the truth, instead of being tricked? If the dead have a habit of getting out of control, that would be an excellent reason not to raise servants or armies, but it doesn’t explain why the necromancers are calling to them at all. Does it take an iron will, a strong mind, a strong sense of self, a strong sense of reality, to command the dead? All of them? Something else? If it yields great benefits, why isn’t everyone a necromancer?
I think most fantasy authors, who can build a system of magic in dazzling working detail, don’t want to think about what happens when that system breaks down. Or they don’t want to take into account that their protagonists are other than perfect heroes, who would never, no never, try to enslave anyone, even someone who is dead.
By the lights of that culture you’re writing about, though, is it enslavement? It might be in our world. It might be in Generic Fantasyland. But you will have to show me why, if the necromancer can safely and cheaply and profitably control the dead, he’s not doing it. Moral concerns have to be expressed to matter to the story. If the dead are utterly mindless bodies, tied only to a shred of spirit that doesn’t remember who it was or know what it’s doing, then moral concerns could be believably much less than in a world where the dead remember who they were and hate someone rousing them from the darkness to use them for information or service or fighting.

4) What is the afterlife like?

Have a clear picture. When necromancy’s involved, the author often shows a contradiction: a rather happy, fluffy, green-fields and singing-lambs paradise is identified as part of the religious beliefs of the common folk, but the dead all describing the afterlife as dense and dark, peaceful, a sleep they don’t want to come back from. (See also point 5).
Here, I think, is a case of a typical Fantasyland afterlife portrait colliding with the typical Fantasyland portrayal of necromancy. Reconcile them. It would be entirely possible to say that the dead don’t want to be summoned from dancing in the fields and feasting with the gods. However, to have them speak of that as “sleep” is really strange. Perhaps the dead have a reason to lie and trick mortals, but it would have to be a really good reason. And if they give inaccurate information about the afterlife, why would they tell the truth about anything else? The necromancers who summon the dead mainly to hear the future or what’s going on in a different part of the world would no longer have a reason to summon them.
Also, what happens when a young necromancer who believes in the Field of Fluffy Bunnies hears about this peculiar place of motionless sleep from the first dead person he talks to? How does he maintain his belief in the Field of Fluffy Bunnies? Are there two distinct kinds of afterlives? Do the dead go on a journey to the Field of Fluffy Bunnies, and only the ones who are recently departed on that journey can be summoned back? (There’s a good reason for the dead to be pissed off: they’re going to heaven, and they don’t want to be yanked back to retrace their steps). Does every necromancer mentor explain the apparent contradiction to his student? Once again, a question like this could lead to a good source of plot tension or a mystery to be solved, but not explaining the contradiction between believed-in afterlife and apparent afterlife is moronic.

5) What is the effect of necromancy on religious faith?

Here are people who know that the dead survive after death, because they’ve talked to them, seen them, maybe touched one or raised an undead army. The dead, if they can be trusted, might even describe the afterlife to them. How do necromancers relate to the faith-based religions that seem to dominate a lot of fantasy?
Here is where you, as the author, have to do some fancy footwork. Most authors don’t want gods interfering in their world. They would upset the plot and put deus ex machinas on everything. So the gods are distant, and the common fantasy person’s way of relating to them is described as “belief.” That implies faith, a notion that the god is watching you even if you don’t receive signs at every turn that yes, he is, and incidentally he knows that you peed on the haystack behind the barn.
Yet necromancers have knowledge. With knowledge of the afterlife, the dead, perhaps even the gods themselves, there is no need for faith. There’s especially no need for faith if the version of the afterlife that the necromancers know about is very different from what the common person believes happens to them after death.
So why haven’t the churches disbanded? Why isn’t the relationship of the common people to the gods based on knowledge instead of faith, and why is death regarded with so much dread and fear of the unknown if they do know?
Perhaps the necromancers have protected the secret. Why? You need a good reason, since people who go into death for the sake of magical wisdom wouldn’t seem to have natural inhibitions about spreading that wisdom to others. The “things man was not meant to know” gimmick is overtired and needs to be done just right to work, since so often the grand secrets don’t seem to be all that grand, the way most fantasy religious doctrines are simple things at the heart of them. If the churches are powerful enough to make the necromancers afraid, why are they allowing mages not under their direct supervision to do all this research into death in the first place? (Perhaps they’re not. Perhaps the necromancers are part of the church, and calling on the dead is part of the worship of that particular god. I’ve seen that mentioned in fantasy, though never deeply explored; the priests of the undead gods who do it are usually the Bad Guys).
If the afterlife the necromancers find out about is the same one that people believe in, there’s no reason that religion has to be destroyed. Yet it should change if it’s faith-based and if the necromancers make their work known. That’d be a fascinating story to read, really.

6) Do not make your necromancers think they are Evil, even if they are the villains.

I am so tired of this. The author starts a scene that could be genuinely terrifying. The necromancer is summoning the dead. Rotting bodies are rising up around him. He’s gesturing with his hands to make them arrange their flesh in such and such a way, a master artist at work. If he slips one step to the left or the right, the dead will consume him.
And in the middle of that he thinks something like, “The village would tremble and suffer before his power. Terros uttered a wicked laugh. They would be sorry they had driven him forth from his home! He would show them the power of evil!”
Please. Who thinks his own laugh is wicked? Who thinks that what he’s doing is evil? Maybe if he’s insane, yes, but insane bad guys are far, far too overplayed. I would also like to know how, if necromancy is so rigorous and exhausting a form of magic, an insane mage would have the concentration or the ability to advance very far in it.
Done right, a scene from the villain’s viewpoint will be exactly like a scene from any other character’s viewpoint. He thinks, he plans, he has a personality, he doesn’t exist just to set forth weak stratagems- or undead armies- that the hero will easily defeat. The other characters can think of him as a villain night and day if they like, but he won’t think of himself that way.
The reason I mention what might seem a common-sense point here is that necromancers are 90% of the time used as the bad guys. The fascination with death and the fact that many fantasy authors think there’s a universal moral prohibition against disturbing the dead- there might not be, in a made-up culture- combine to render them so. Some authors apparently see nothing wrong, even when they characterize other “villains” as complex, with making the necromancer a one-dimensional caricature. I’m asking you now to apply the same level of thought to him as to anyone else. Just because you hate him doesn’t mean he hates himself.
Rant the next shall concern ghosts.