This is a single target, destro / resto, stealth / ganking build that avoids Merciless Resolve and is not overly reliant on heavy attacks. I make no claims that this is the best build ever. It is what I have enjoyed playing, coming from a stamina build. I had real difficulties finding a Magicka Nightblade PvP build that I enjoyed prior to putting this together. I mostly play in IC. It is quite easy to play and easy to obtain the sets, with one craftable and one overland (farm or buy in a guild store). As you can craft a sharpened destro staff, it avoids the huge cost of a sharpened Spinner's destro staff, or the effort to farm some other staff, such as Amber Plasm.
Magicka nightblades have a reputation for being hard to play in PvP. Merciless Resolve is one reason for that. You are a stealth / ganking class, but your big weapon is not available from stealth. It is clunky, it is telegraphed, and it only becomes available as your gank turns into a duel. Once that happens your chief advantage is arguably lost, as Magicka Nightblades tend to be lacklustre for damage until the finnicky Assassin's Will proc.
For single target fights your best opener is probably an Inferno staff heavy attack. If you want to maximise that, this brings another set of problems, such as:
* The setup time is very slow, especially when you count buffs like Merciless and Magelight for the Empower.
* You don't reliably get the crit from Shadowy Disguise, since the heavy attack charge time is increased in Homestead and just about the same as the Shadowy Disguise buff time. A reliable way to guarantee the crit is to re-cloak after the heavy attack. It's the cloak AFTER the heavy attack that guarantees the crit when the attack lands. Perhaps running up close to the target also solves the issue, as this reduces the heavy attack travel time. However this impedes your natural playstyle, which is ranged.
* To maximise heavy attack damage, you sacrifice other attack damage via CP (Staff Expert) and sets like Elegance or Stygian.
This build does not optimize for heavy attacks. It does not aim to one-shot people from stealth. You will have a (hopefully short) fight after the opener.
RACE
Breton. Other races work as well, of course, but Breton has an impact in so far as the cost reduction passive helps with perma-cloaking out of combat. See further discussion below.
GEAR
5x Way of the Arena
5x Spinner's
1x Monster helm or shoulder of your choice (for example Molag Kena, Bloodspawn)
5x light, 1x medium, 1x heavy armor.
All Magicka enchants on the armor.
All spell damage enchants on the jewelry.
Gear traits: Mostly Impenetrable. Some Divines or Well-Fitted also work.
1x Inferno staff, Sharpened
1x Resto staff, Defending
Weapon enchants of your choice. I currently use the raise weapon damage enchants.
All items at least purple quality, except weapons should be gold quality.
If you have access to a vMSA or vDSA weapon, you might use that instead of the monster helm / shoulder.
WHY WAY OF THE ARENA? WHY BLOODSPAWN?
I come from playing a stam DK. My DK build uses immovable and speed potions, as well as Forward Momentum, which gives you 8 seconds of snare and root immunity. Coming from that build to Magicka Nightblade was a shock. You get CCd, snared, rooted, out of stamina in no time, unable to act. This prompted me to look for a set that would mitigate this problem. Of course Reactive Armor is probably the better known set, but it is a heavy set with healthy jewelry. I wanted a set that you can craft for a 5/1/1 light armor build and for damage and perma-cloaking, see below. Along with CP, Way of the Arena cuts Break Free cost in half. Another way of looking at this is that you have doubled your effective stamina pool from 10K to 20K for breaking free, and you've doubled your effective stamina regen from 600 to 1200. This really does work. I find that I am much less likely to be helplessly CCd and out of stam with this set.
I used one piece Molag Kena when I started, but am now trying Bloodspawn, raising actual stam regen to 800 and effective stam regen to 1600. Thus, if I do find myself CCd and out of stam, it doesn't take "forever" for stam to come back and break free.
An alternative is the Amber Plasm set, however I did not find it as effective. You don't really need the magicka regen, you have to farm it, and so on. The boring stats of the Arena set (Health, Magicka, Stamina) are actually all useful for this build and allowed using all Magicka enchants.
MUNDUS: Thief
DRINK: Witchmother's Potent Brew
POTION: Detection / Spell Power / Magicka
SKILLS - DESTRO BAR
Swallow Soul
Mass Hysteria
Flame Reach
Impale
Dampen Magic
Soul Assault
SKILLS - RESTO BAR
Concealed Weapon
Siphoning Attacks
Efficient Purge
Healing Ward
Shadowy Disguise
Any Ultimate of your choice
WHY THESE SKILLS? WHAT ARE THE ALTERNATIVES?
Concealed Weapon, Efficient Purge, and Shadowy Disguise all work together to facilitate stealth. Concealed Weapon is there solely for the speed buff. It is a great shame having to slot this, but I find it very useful for keeping out of harms way and not be detected while stalking players in cloak.
Efficient Purge is there for when you want to escape and have difficulty re-entering cloak because of effects placed on you, or if you are snared. It is not a high health build, so I went with this morph over Cleanse.
Shadowy Disguise is cheaper than Dark Cloak at rank 4. If you are a Breton, with 100 CP in Magician (cost reduction), you will then require the Atronach mundus stone OR a drink (e.g. Witchmother's) to have just enough regen for perma-cloaking out of combat. Note that magicka regen enchants on rings or from item set bonuses DO NOT influence your OUT OF combat regen and therefore DO NOT help you perma-cloak. You also require at least 5 pieces of light armor. If you are a different race, you may require 6 pieces of light armor or a cost reduction enchant or both Atronach and drinks. This build does not require a potion to perma-cloak. You may use one, however, to recover magicka while perma-cloaking.
All of the above could arguably be replaced by a single skill for combat purposes: Mist Form. However I did not want my character to be a vampire.
I do not see a way around Siphoning Attacks. I find you have to slot it in PvP for the stamina sustain. You also want a Siphoning skill on this bar for more magicka, and therefore bigger Healing Ward.
Healing Ward is there for obvious reasons. The only other heal in this build is Swallow Soul.
If you are a Vampire you might use Devouring Swarm on the resto bar. I have been switching between Soul Harvest, Soul Tether, and the resto ult, but I basically never use the ulti on this bar.
When it comes to the destro bar, the first thing to keep in mind is that you want at least one ability from Siphoning, Assassination, and Destro Staff to optimize damage and shield size. Ideally you also want one or two Shadow abilities to balance out the health with the other bar. This constrains your skill choices quite a bit.
Swallow Soul. Your spammable. Never underestimate healing while attacking. Choose Funnel Health, if you prefer. The obvious alternative is Crushing Shock / Force pulse, which is good against flappy DKs, but you just knocked your only Siphoning ability off the bar, so replace Flame Reach with Agony or Crippling Grasp, if you're doing this.
Mass Hysteria. If stamina player(s) are wailing on you, this easily beats Flame Reach. Whereas on my DK I am very happy with Fossilize (a ranged single target abaility, like Flame Reach), my NB tends to be snared or immobilised and much more vulnerable. The AOE from Mass Hysteria counters players quickly, even when they are behind you and you have trouble turning. Then there's the maim, snare and increased resistances. Too good to pass up.
Flame Reach. You have to put a destro ability on the bar, the damage buff is too good not to. It was between this and Elemental Drain for me. Flame Reach is good against players trying to flee or reposition. It also works with a heavy attack, as part of the stealth opener.
Impale. Your Assassination ability.
Dampen Magic. I find that Healing Ward alone isn't really enough. I previously had Magelight slotted. It would be nice to have the crit, but I find the shield necessary.
Soul Assault. Cheese kills. This is what gets you kills and what makes the build easy. With Major Sorcery (from a potion), my tooltip is about 70K. This build also has 14K penetration. I've killed quite a few players outright from stealth with this. Good and alert players will counter it. Many don't.
NOTABLY ABSENT
Merciless Resolve. Any stealth ganker would want this for the Minor Berserk, right? The way I understand that buff, it is supposed to give you an 8% damage increase on actual damage, NOT just your spell damage parameter. Well, it doesn't. In tests on my target skeleton it gives merely 5%. Basically I hate this skill, and the buff isn't even as good as it says it is. Dismissed. Wish it worked more like a Fighter's Guild Ultimate, with a permanent damage buff while slotted and available indefinitely, once charged.
Shadow Image. Did I mention I wanted an easy to play build? Must learn to use this some day.
OPENING FROM STEALTH
You have a couple of options. As I mentioned, the surprise of getting Soul Assaulted from cloak makes opening with that stronger than it has any right to be. It can be an "I win" button, but it can also go horribly wrong. You don't want to be too far, or they'll LOS you or run out of range. You don't want to be too close, since that makes it easier for stam players to counterattack and kill you. They can also just block, be too tanky, cast a shield or heal.
Option two is, you simply start weaving Swallow Soul (opening with a heavy). I like this, because you get a feel for how tanky they are. Once their health has dipped, only then Flame Reach, Soul Assault, Impale.
Option three is a heavy attack + CC opener. Cloak, swap bars, hold the heavy attack button, press Flame Reach: Heavy Attack and Flame reach will go off together, when charged. I haven't done this often enough. I have a feeling it may, in fact, be better to withhold the CC until you have gauged their tankiness and only use it before Soul Assault instead.
PC EU: Magblade (PvP main), DK (PvE Tank), Sorc (PvP and PvE), Magden (PvE Healer), Magplar (PvP and PvE DD), Arcanist (PvE DD)
PC NA: Magblade (PvP and PvE every role)