Gear Review
Our goal is to stack as much Weapon Damage as possible, one may argue for Balorgh but the playstyle of waiting for Toxic Barrage is a bit too reminiscent of how one would build Ultimate for Soul Tether or a Destruction Staff Ultimate on a Magicka Nightblade. Furthermore you should be working to kill people based around your cooldown on Mechanical Acuity more so than your Ultimate if that's your playstyle. If not, you can save for an Ultimate and opt for 2x Balorgh.
Mechanical Acuity is the de facto primary set due to its guaranteed proc for Critical Damage. With this set we can shift away from trying to go for Weapon Damage AND Weapon Critical rating which is why we don't need to look at sets or skills or potions that offer Major Savagery or Critical bonuses.
Your secondary set is up for debate. I've found good results with Spriggan's Thorns due to the fact a lot of people have been trending towards using tankier setups or the Pariah set which offers more Armor gain the lower their Health. There are alternative sets to consider such as Swamp Raider, Archer's Mind and Heartland Conqueror which can also work well.
Although Heartland Conqueror provides very very slightly lower amount of Penetration compared to a 5x Spriggan, it may be beneficial to run Heartland due to its Max Magicka bonus which you will surprisingly find is very needed on a Stamina Nightblade ganker (Cloak usage).
Titanborn is also a good consideration as you're running Simmering Frenzy which could potentially synergize well with the set's added Weapon Damage and Penetration bonuses offered by being under 50% Health.
Vateshran Bow is also a consideration but this requires you to shift to replacing the 1x Velidreth/1x Balorgh with the last 2 pieces of a 5x set (3 jewelry + head/shoulder Spriggan, 5x Mechanical Acuity on body and Vateshran Bow frontbar as an example) while also considerably putting you at a positional disadvantage if you want to try to gank at a closer distance.
However, if you'd like to gank at a closer distance, 5x Deadlands Assassin could be a strong consideration with or without Vateshran Bow. But if we're going to consider melee ganking, well that would probably have entirely different setups/gear, perhaps consider a Two Hander setup with other skills.
Your backbar is up to you, some setups will prefer an item set that gives a 2x bonus of Stamina Recovery, Weapon Damage, Max Magicka or whatever suits your playstyle. You still need a Two Hander on the backbar however so you
Skills Review
Your basic rotation will consist of prebuffing Channeled Acceleration, Rally and Siphoning Attacks in stealth then activating Simmering Frenzy on the backbar. The longer you let Simmering Frenzy be active, the more Weapon Damage it grants you. Conversely, it starts draining your Health at a larger amount every tick so popping a tri-pot + recasting Rally before you start your frontbar rotation is an option for a larger amount of Weapon Damage.
Speaking of your frontbar rotation it's usually: Shadowy Disguise > Snipe > Light Attack/Medium Attack > Crushing Weapon > Poison Injection. By the time your Light Attack reaches the enemy player, your Crushing Weapon will have applied to that Light Attack.
There are various alternatives and things that we tested. Focused Aim could be the morph alternative to Lethal Arrow, in particular if you want the Minor Breach/more Armor Penetration. However if you're using Swamp Raider, Lethal Arrow will probably be a better option. Either morph will hit extremely hard when you're close to 9,000+ Weapon Damage.
We also tested Silver Shard versus Crushing Weapon and Crushing Weapon is simply better in terms of the damage following the Snipe. Camouflaged Hunter is nice especially when you're flanking your target from somewhat of a distance but Arrow Spray is a really nice skill to use against other gankers or to simply finish off a Nightblade that tries to go into Stealth/Invisibility right in front of you.
Toxic Barrage and Flawless Dawnbreaker are your go to Ultimates following your Snipe. Depending on your distance to target Incapacitating Strike is an option as well.
Your backbar will have two healing abilities including Resolving Vigor and Rally (which provides the Major Brutality to increase your Weapon Damage) along with Temporal Guard, the Psijic Ultimate morph that provides Minor Protection.
Many will also ask why I don't slot Piercing Mark or a form of Major Breach. The simple fact is that once an enemy player is marked or knows you're in the vicinity, they start buffing up or blocking or Roll Dodging. As a ganker you don't want to engage a player in a stand-off, you want to kill a player unexpectedly.
Relentless Focus was also tested but it was either too bothersome or nigh impossible to try to keep the 5 stacks/stay in combat or there wasn't enough room on the front bar.
Siphoning Attacks will be used over Leeching Strikes as Magicka is in a huge demand for getting away or having a guaranteed invisibility with Shadow Disguise.
Craft
Warfare
Fitness
Champion Points Review
We want to go for as much damage as you can. There are three separate benefits to going with Backstabber:
1) It procs your Camouflaged Hunter for Minor Berserk.
2) The guaranteed Critical Damage from Mechanical Acuity stacks with Backstabber and Fighting Finesse.
3) Most players have their cameras pointed towards where they're going, they rarely swivel cameras around or look behind them as they engage others or ride long distances. Attacking from out of their view can add an advantage.
Mitigation is not needed, just sustain and damage as getting caught out by other gankers or groups of people will get you killed anyway. Positioning and careful timing will be of more benefit to you in the long run.
Introduction (Update 30, Blackwood)
This build is intended purely for ganking (stealthily ambushing) players in open world PvP (Cyrodiil or Imperial City) and hopefully one or two shotting them when their defenses are down. Like its Magicka Nightblade bomber counterpart, positioning, prebuffing and timing are actually surprisingly more important than what most people on the receiving end of a gank would assume. This is particularly more so when players are trending towards building for tankier setups.
While any race could work, the recommended race for this in terms of min-maxing the guaranteed Critical Damage would be Khajiit.
Recommended Mundus Stone is The Shadow.
Recommended potions for this build will usually be Essence of Health (tri-stat potions that give Health, Magicka and Stamina).
Recommended food for this build is Bewitched Sugar Skulls or Orzorga's Red Frothgar if you want to raise your Stamina attribute points more while gaining Magicka Recovery for Shadowy Disguise.
Vampirism is a must due to its various skills and passives that mesh incredibly well with the stealthy and high-burst damage playstyle of the build.