There are three necromantic base classes of note: the Wizard, the Cleric, and of course the Dread Necromancer. There are a number of other classes capable of using Necromancy (Sorcerers, for example), but they rarely do it well and often fail in surprising ways. Many of the signature Necromancy spells are used very rarely (Create Undead is not a spell to be used every day, or even during adventures generally – it’s a downtime spell) and the vast majority of characters with “spells known” are completely unsuited to necromancy in the traditional sense. Spell preparation is definitely the way to go, with the notable exception of the Dread Necromancer who has such a large list of spells to spontaneously cast that she might as well have a spellbook. There are some surprisingly good spells nominally of the necromancy school on the Druid and Wu Jen lists, but as they are not thematically related to what we think of as “Necromancy”, these spellcasters will be dealt with elsewhere if at all.
Becoming a Necromancer late in life, by entering a prestige class that provides spellcasting is nominally an option as well. Unfortunately, the most flavorful classes (such as Blighter and Death Delver) are pretty universally terrible, while the actually effective classes are either so obviously broken (Ur Priest), confusing (Chameleon), or both (Beholder Mage) that DMs won’t actually let you play them. There are to my knowledge no playable builds that involve these classes. Between the pain of “catching up” by crawling through low level spellcasting when facing monsters of medium CR, and the tremendous inertia of DMs as regards being forced to learn the rules of an entire new spellcasting class, these paths are a non-starter in almost all cases.
The Necromancer WizardEdit
Wizard is a class that is 6 levels long (unless you intend to take the Planar ubstitution level at 10th, in which case the class is 10 whole levels long). Necromancy as a wizard is a surprisingly hard road. The first thing to realize is that you do not have an army of the dead! If you wanted an army of the dead, you’d be a Cleric or a Dread Necromancer. Wizards have bonecrushingly powerful necromancy at their disposal, but almost none of it has anything to do with having a shambling army of animated corpses following you around. Necromancy from the wizard perspective is usually about the Soul, and is a deeply powerful school centered around Fear, Possession, and more recently – Cold.
Hilarious note: All the Unearthed Arcana Specialist wizard necromancy trade-ofs are terrible. Except the Enhanced Undead, which gives an unnamed (and stackable) +2 hit point per hit die bonus to all the undead you make. You can benefit from that even if you one-level dip into Wizard.(as an additional note do not forget to stack corpse crafter so your undead are even stronger) giving them an addition enhanced undead effect
Clerics are better than WizardsEdit
At its core, the Cleric is a better class than the Wizard. It gets better armor and weapon proficiencies, better saves, more spells per day, more hit points, the ability to ignore ASF, free knowledge of the entire spell-list, and a better BAB. That's not to say that any particular Wizard is outdone by any particular Cleric, there are some very powerful spells on the Wizard list that are not on the Cleric list. But if a Wizard finds himself casting a spell that's on the Cleric list, at least for that round he's the big sucker.
So while you can certainly make a serviceable Wizard who happens to focus on Summon Monster, this character is going to be inferior to a Cleric that does the same thing. Even more so because there are domains like "Summoning" that give Clerics powers in that field that a Wizard can't duplicate. Such a Wizard character may be a vital member of whatever team he's on, but the fact is that there's a guy in the corner singing everything you can do. Like taking the 7th level of Fighter, it may be "good enough" for your game at home, but it's objectively inferior to other things and will not be taken seriously as a choice here.
Of course, Wizards are in an even worse position vis a vis the Cleric in the arena of animating corpses. While command undead is quite competitive with rebuke undead, and Animate Dead apparently works the same whether you are a Wizard or a Cleric - that's an optical illusion. Clerics get access to Animate Dead early. And they get access to Desecrate at all, which means that all their skeletons have 2 extra hit points a level that the Wizard can't match for some time (remember to construct an altar to Nerull everywhere you want to make Undead, because it doubles the Desecrate bonuses and doesn't cost anything). Finally, there are domains that give real bonuses to your necromancy that you can have if you aren't stuck shelling out for the worthless Death Domain because you are trying to get into True Necromancer (Boo!).
A real dead animating Cleric can have the Deathbound Domain, which gives her an additional 50% to the skeleton hit-die cap. That means that a Cleric's army of the dead at 5th level is bigger and better than the Wizard's army of the dead at 7th level. And using those spells didn't even take up valuable space in a spellbook or anything.
The Necromancer ClericEdit
Cleric is a class that is potentially as much as 12 levels long, but is actually quite variable in length. Clerics are the default necromancers from the standpoint of the Skeleton Army. It is normally required that you forfeit your ability to heal the party by being a Cleric Necromancer. It’s not like you’re going to prepare a cure spell – that’s crazy talk! However there are some obscure loopholes you can exploit here. Nothing bad happens to Clerics if they cast spells with an opposed Alignment to their own, it’s just that such spells are not normally on their list. A Cleric who gets Undead Animating off of their domain lists or similar other sources can jolly well cast those spells even if they are Good. So if you happen to be a Lawful Good Cleric of Wee Jas, you channel positive energy but can still animate the dead (because you get all those Death Domain spells).
The Dread NecromancerEdit
The Dread Necromancer is a class that is 20 levels long, though it is recommended that you opt out after your 8th level because the return diminishes there after. Early in its life it is a melee warrior, and later on it’s a passable Undead Leader. You can cast any spell on your spell list as a spontaneous effect, which is a unique way to do things and of course is the most favorable spellcasting mechanic ever. Adding spells to your list is easy and fun, the default method is to use Arcane Disciple, which adds 9 spells to your list each time, but more elaborate methods (such as a Ring of Theurgy) exist. All Dread Necromancers, regardless of their ultimate goals, have Tomb Tainted Soul as their 1st level feat. That’s not a recommendation, that’s a simple fact. The ability to heal yourself with your own touch is invaluable, and in the long run you are going to have the ability to spray negative energy all over everyone within five feet of you, including yourself. If you don’t have Tomb Tainted Soul, your negative energy bursts are a suicide bomb, if you do have it they are instead an vampiric healing attack.
Other Spellcasting ClassesEdit
There aren’t just 3 spellcasting classes, there are just 3 spellcasting classes that are good. Many times a player may be tempted to play one of the many other spellcasting classes that dot the landscape and we can say without the slightest shadow of a doubt that under no circumstances should any of them be used. For example:
Don’t be an Archivist:Edit
Archivists don't have Domains or Rebuking, and they don't automatically know all the Cleric spells. So unless something weird happens, they are in all ways inferior to a Cleric at Necromancy, or anything else. That weird thing, of course, is that as an Archivist you have the ability to have the DM allow you to find powerful and unique spells that you can scribe into your book and rock the house with. But these spells aren't under your control. They fall into your lap because the DM puts them in your lap, and not otherwise.
So there aren't really any Archivist "builds". It's just a "maybe the DM will give you some cool things to do" class. Like Pun-Pun, the power of any particular Archivist has nothing whatsoever to do with its own intrinsic abilities, it is entirely based on whatever the DM felt like forking over out of pity because you couldn't do anything good on your own. And when it comes down to it, Wizards already have the "The DM can give you additional awesome spells by dumping magical writings in your lap" power. And they are independently good. So no, we won't give any examples of Archivist builds, because they aren't independently verifiable.
Don’t be a Warlock:Edit
Warlocks get access to an Invocation called The Dead Walk, which has two uses. It can either work like normal Animate Dead (that you pay full price for), or it can act as a special kind of Summon Undead that has a duration of several minutes and has a material component of one corpse. That's weird, but it's not particularly good. If it was backed up with anything else along these lines it might be worth thinking about, but it isn't. Warlocks get no desecrate, and no special Undead leadership. They don't actually have any cool powers at all.
Yathrinshee, the Dark Beauties of Bad Class FeaturesEdit
Like the True Necromancer, this is an arcane/divine combo PrC that is very cool and we'd totally play it if it wasn't completely crippled. Aside from the class requiring you to be a totally hot necromancer dark elf chick, it wants you to have five levels of Cleric of some god you don't care about and three levels of Wizard. Then, over the course of 10 levels you lose FOUR more caster levels from both your classes. Add in your level adjustment for being a drow, and at 20th level you are...wait for it...NINE levels behind in your cleric spells and ELEVEN levels behind in your Wizard casting. You can't even try to mitigate this with Ur-Priest cheese, since you need to follow this one god to get into this class.
Their one great ability is the Curse of the Revanancer, which is awesome, as it lets you kill things with spells and they automatically become zombies under your control. Thats cool, and even very powerful under the right circumstances, but how are you even supposed to kill things with your spells at any point in your career?