Firstly, apologies to Lost_Map, but your guide was taking too long to come out!
I would like to start out by saying this guide is mostly opinion, based on my time playing, testing various things, intuition as an RPG enthusiast, as well as snippets of information I've picked up around these forums. I am by no means an expert on the game or the class in specific but I do think I learn relatively quickly and a good deal of the information I have learned thus far could be of great use to new players. If you think you have better ideas for some areas or straight up cold-hard facts and math that disprove things I've said then by all means feel free to post here or message me and I will make adjustments as needed.
Overview
Necromancers are dark spell casters that focus on reanimating the dead and commanding them in battle. They also specialize in damage-over-time spells, which are the best among all classes. They have the best fear spells in the game and the ability to explode monsters, whether they are alive or dead.
Class orientation: Damage-over-Time Spell caster, Pet, Cloth Armor
As for Races, Necromancers can choose from Human, Erudite, Dark Elf and Gnome. I will go more in depth on Races in a later portion of the guide.
The Necromancer Starts with +5 Dexterity, +5 Wisdom and +15 Intelligence, regardless of race. I will go more in depth on Stats in a later portion of the guide.
Races
The most important factor your race will have on your character and how you play it will be their racial ability, or in some cases, passive. Stat-wise, the race choices are largely inconsequential in the long run. If you are into min-maxing, however, here are the facts.
Human - The Race with the lowest Intelligence but a very powerful Racial Ability. Possibly one of the strongest Racials in the game, Second Wind gives you a quick regen to health and mana as well as a boost to damage dealt and reduction to damage taken on a short cooldown.
Erudite- The Race with the highest Intelligence but a lackluster(in my opinion) Racial Ability. Chromatic Aegis makes you highly resistant to all schools of magic on a short cooldown.
Dark Elf - Race with the second highest Intelligence and also with a very powerful Racial Ability. Sanguine Torment causes you to leech health from all damage inflicted, easily bringing you back to full health from the brink of death and making you nearly unkillable for the 12 seconds it is active as long as you don't stop gatlinging out those Fireballs!
Gnome - A race just shy of tying for second highest Intelligence by 1 point and fairly strong Racial Abilities. Warp Field can both save your life and also make chaining mobs together much easier. Silence Immunity is also immensely powerful and coupled with the Talent that makes you immune to Fear you'd only need to worry about Stun Reduction as a Gnecromancer.
Verdict: All four races have their pros and cons but I'd say, in my opinion, that the Gnome and the Dark Elf are probably tied for best over all. Go with what your little RP heart says, though, if Min-Maxing isn't a concern for you!
Also of note, as far as Sex is concerned, both are relatively close. The extra Stamina from Male and the extra Regen and to a lesser extent Wisdom from Female both have their place but in the end these bonuses are a drop in the bucket so feel free to choose what you prefer.
Stat Weights
I will generally grade stats that I take the time to list on a scale from Okay to Good to Very Good, Okay meaning it's a decent enough stat but don't let item decisions completely hinge on it, Good meaning definitely grab this if you can and Very Good meaning drop everything for this stat! Stats I don't list are negligible for the Necromancer. Again, this is all opinion through testing and play.
Main Attributes
Intelligence: Increases your spell damage and your mana pool. Also allows you to level skills faster. Very Good Stat.
Wisdom: Increases Magical Mitigation as well as the effect of healing abilities which comes into play with a few Necromancer Skills. Okay Stat.
Stamina: Increases your Maximum Health. Okay stat.
Offensive Stats
Critical Chance: Increases how often your spells hit for extra damage. Good to probably Very Good Stat.
Critical Damage: The amount of extra damage your Critical Hits do. I believe, but am not 100% sure, that this caps out at 150%. Very Good Stat all the way to cap as it directly correlates to a very strong damage increase.
Casting Haste: Effects how fast your spells take to launch after you cast them. How quick you fire off them Fireballs! Very Good Stat
Skill Haste: I don't believe the GCD effects casters very much, if at all. If it does, it is not at all noticeable when spamming spells as even a shortened cast bar will generally take up the time the GCD would normally have. Probably not something to worry much about, though I have roughly 50% currently passively somehow, which may be the cap and also may be why I don't notice it.
+Skill abilities(I.E. +Evocation, +Conjuration, etc) and +%Magic Damage/+%All Magic Damage, +%Fire Damage, +%Poison Damage - I feel like these Stats, especially when combined on an item, are very strong but I don't know specifically how strong. If it comes down to it, I think it might be best to ditch these for the more uniform Offensive Stats and the Main Attributes. If anyone has done any math with these, it would be much appreciated. I rate them as Good for now. Also, +Evocation is obviously stronger when you switch to spamming Fireballs and +Conjuration is stronger if trying to utilize a DOT build as those skills correlate to those spells.
+All Skills is also probably a very solid Stat wherever you can find it.
Defensive and Recovery Stats
Resists - Seem very strong and very important, especially when dipping into Nightmare Difficulty and even more so going forward into Hell, both of which I'm fairly certain you receive some sort of Percentage Penalty to Resists simply by stepping foot into them. I'd rate them at a Very Good level on a per-case basis, I.E. Equip a bunch of Cold Resist gear when facing off against Maelentia, etc.
Reduced Duration Stats: These seem very strong, especially Stun and Silence Reduction. Fear reduction, however, is almost entirely useless as there is no reason not to take the Howling Terror talent. I'd say at least a good portion of Stun/Silence Reduction is actually required. Ignore Silence Reduction if you're a Gnecromancer! Very Good Stat rating.
Life Leech -I feel this is the only Health Recovery stat worth getting, but I'm not even sure it's a particularly big deal for a caster. Likely depends on Casting Haste. Good Stat for sustainability.
Dodge Chance, +Defense, +Physical Reduction, +Magical Reduction - Similar to their Offensive counterparts, I feel like these Stats can be very powerful, especially in tandem, though I'm not 100% sure on the precise potency. Worth snapping some up at the very least with an Okay Stat rating.
Mana Leech, Mana Per Tick, Mana On Kill - I don't find Mana Recovery to be much of a concern, personally, though that may be because I have an okay amount on gear, especially the 2 and 3 Piece Wraith bonuses. Honestly I feel like Detonate Soul is a dead ability because Mana is such a non issue. Okay Stat
There is a caveat with Stats. As much control as you'd like to have over this it all comes down to what RNG has in store for you. You will utilize the best of what you get, generally speaking, and luck will play a huge part.
Leveling
Some classes that I've played seem to follow a fairly straight-forward, linear path to cap, Talent and Skill usage wise. I don't feel that the Necromancer does, however. I feel that the Necromancer has a transitional skill usage set up that is more optimal until you reach certain Talents in the tree.
Prior to capping Talents such as Howling Terror and Corpse Explosion I feel that it is best to start out a pull by throwing up Cascading Darkness and Ignite Blood followed by your general Bone Spirit spam, chain and then, as the first enemy dies pop a quick Corpse Explosion on the new friend. Up until probably Nightmare you can also be a little more careless with your chaining, sometimes even filling the screen if you're ballsy enough, crowd controlling with Howling Terror and your Single Target fear, Invoke Fear, if things get hairy. This changes, however, when you reach certain Hallmarks. Eventually, namely once you cap the Howling Terror Talent as well as getting the Asystole spell and capping the Corpse Explosion Talent you will transition to almost entirely abandoning Cascading Darkness and Ignite Blood, simply tossing out Asystole followed by righteous Fireball spam. I will go a little more into detail about all this in the Talent, Play style and Advanced Tactics sections of the guide.
As far as zones are concerned, I've found that Galeblast Fortress 1 is a very solid zone to farm for the Necromancer Build I've outline here. The mobs are relatively safe to chain even on Hell Difficulty, unlike a lot of the zones going forward which contain Giants, naturally Fire Resistant enemies and a lot of enemies who silence and stun which can be annoying and deadly. Something you can chain makes a big difference on Experience gain and Magic Find.
Gearing
Now, generally you're going to go for whatever pieces you can find that have nice heaping-helpings of our preferred stats of Intelligence, Critical Hit Chance, Critical Hit Damage, Casting Haste and the other strong contenders I outlined in the Stats section of the guide. Keep an eye out for any well rolled Rare(Yellow) items to round out slots that you haven't found a strong Unique for yet.
Weapon Choices:
I'm not entirely certain as to whether or not a caster Piercing weapon and a caster Offhand are better or if a nice solid 2HB staff is but from a lot of what I've gathered around the forums Staves are very powerful with immense Stat amounts and rolls so it's likely that in the End Game a powerful Elite Unique 2HB will come out on top as Best in Slot. I currently haven't found a decent one so I'm sticking with a Dagger/Offhand Combo.
Some Recommended Pieces
Wraith's Set
The Necromancer's class set. Mostly lackluster set bonuses up until the Six Set Bonus which seems to be absolutely absurd, though I have yet to obtain the full set to test(Chest... please drop
). All of the indivdual pieces seem fairly strong, especially the ones previously listed as "Elite" for obvious reasons, and are generally worth equipping by their own merit. 
Some honorable mentions are several of the other caster sets, namely the Wizards Malefactor's Set and the Mages Summoner's Set, among a few other select class set pieces, which have very powerful Set Bonuses, even for our Necromancer.
I will update this section as I gain more knowledge of the vast amount of gear in the game
Again, I'd like to reiterate the Stats caveat here as Stats are about 99% tied to gear(the other 1% being Race/Sex choice). Most of this will come down to what drops for you.
Talents
The first talent I max out is Bone Spirit. In Normal Difficulty you have a lot more freedom to work in extra damage at the cost of survivability and this Talent goes a long way to that end, buffing your main nuke by quite a lot.
Next I work towards the REQUIRED Talent Howling Terror. Even if this just gave Fear Immunity it would be required, as that on its own is absurdly strong. However, this also converts Bone Spirit into the spell Fireball, an ability that not only does more damage, but becomes a splash AOE, hitting 2 extra targets adjacent to your main enemy. This ups your damage quite a bit as well as making it easier to do larger group chains at these early levels where, again, you have the freedom to be a little more reckless. Also adds an extra cold DoT on top of that to any Feared targets.
Next I generally work on capping Invoke Fear. Extra damage to Feared targets is a very strong effect, allowing you to burn down Boss/Rare/Champion enemies you have feared much faster. The added chance to proc Fireball(or regular Bone Spirit if you opted out of that talent for whatever reason) upon being hit by an enemy is icing on the cake.
Next you can decide for yourself if you wish to work towards capping either the Asystole or Corpse Explosion Talents. Both Synergize well together. Asystole is probably the better option to start with as 15% Magic Damage when Diamond Skin is active is rather potent. Keeping Diamon Skin active until it is depleted rather than on a timer is a good survivability boost as well, and the bonus +2 to Bone Spirit and Invoke Fear are also very nice.
After capping all these you have a few extra points that I suggest putting into Drain Soul but that's all up to you.
Here's my current build.
Feel free to try out any combination and see what works for you. I may also just be completely missing something on some of these. If so, let me know.
Playstyle
Let's start with the basics. Every time you relog you're going to need to resummon your pet and recast Arch Shielding. Now, I know the description of the class states that the Necromancer is both a Damage Over Time and a Pet class, and technically this is true. However, in my experience, pet damage is negligible at best, even when utilizing pet enhancing Talents. The biggest perk to your pet is that he will tend to soak up damage and harmful status effects, a major boon to a fairly squishy class. Similarly, DOTs are mostly tertiary damage, or a means to an end, such as Asystole helping to proc extra Corpse Explosion triggers.
The majority of your damage will be coming from Bone Spirit and eventually Fireball, followed by Asystole when you get to that point and Corpse Explosions should you choose to weave them in.
Advanced Necromancing
For normal mobs you will mostly just be pulling with Asystole followed by plenty of Fireball spam. Alternately, if you don't care to toss up the DOT you can literally just Fireball spam. When it comes time to pull a Boss/Rare/Champion I tend to wait for them to start to cast spell and then I toss out Invoke Fear. At this point I put up Augment Death to buff my pets damage a bit followed by Bond of Death to keep health trickling in. If your first fear falls off early, such as if they have the Dauntless Affix, as most bosses do, you can toss out a Howling Terror. By the time Howling Terror wears off Invoke Fear should be back up, allowing you to generally keep one mob Fear locked for around at least 45 seconds. If the enemy isn't dead by this point it's time to cycle through Diamond Skin and then Drain Soul when getting low. If you opted to be a Dark Elf you can also use your racial to keep yourself running until another Fear is available should a Boss fight run that long.
A good tactic is also, prior to pulling the boss, to pop Diamond Skin. With the maxed Asystole Talent bonus it will stay on until it is used up. You can put it on and then wait for the cooldown and go into the fight with it on and an extra charge ready to go. If you aren't worried about survivability at all this is also the key to maximizing your DPS. Keeping the boss Fear locked will keep you shielded with 100% uptime, giving you that same uptime to the other portion of the Asystole Talent bonus, 15% to all Magic Damage.
If the enemy is Fire Resistant I tend to weave in as many Corpse Explosions as procs will allow, and occasionally even dip back into things like Cascading Darkness and Even Detonate Soul for a little extra oomph. If the mob is either Poison and Fire Resistant OR Chromatic I tend to immediately attempt Fleeing and/or Feigning Death as those fights are going to just take way too much time that you could be spending killing easier prey.
If the mob isn't Fire Resistant I feel it's best to just spam that Fireball key with Asystole ticking for a little extra DPS
Conclusion
The Necromancer is a fairly powerful class, especially damage wise. Their Glass Cannon-ness is definitely real, though it can be well augmented through intelligent Fear usages as well as healing through Bond of Death and Drain Soul as well as a bit of extra Effective Health via Diamond Skin. I would, however like to see either a shift away from a flat nuke class towards fully viable DOT and/or Pet builds, or at least have those brought up a bit to be competitive. Overall, within the state of the current game I feel the class is perfectly fine for now, though.
At any rate, I hope the guide helps some people, and always remember to have fun. See you in Vandamor!
-Hentai
Lexicon
GCD - Global Cooldown, or when your bars go dark after a skill usage and the length of time they stay that way before you can utilize another ability.
2HB - 2-Hand Blunt, generally a Staff
DPS - Damage Per Second
DOT - Damage Over Time
Nuke - A large, hard hitting spell. Usually spammed.
Min-Max - Eking out every little bit of efficiency from a character as possible.
RP - Role Play
RNG - Random Number Generator. Luck, or the roll of the die deciding the outcome for you.
This guide was written as of patch 1-0-24. More updates to come with content patches and as I learn more.
