Gear Review
It seems the most logical conclusion at this current time that a pure heavy attack damage setup for pretty much all classes ends up in a medium armor setup with Storm Master and the heavy armor set Sergeant's Mail in the form of jewelry and weapon.
It also helps that the main item sets are very easily farmable by players of any level!
With Oakensoul as a Mythic item that restricts other Mythics from being used and also restricting the usage of a second skill bar, we don't have too many options in terms of set availability and also makes us resort to a 1x bonus set such as Slimecraw's 1x Critical Chance bonus.
Unlike the other classes, the Nightblade has a significant amount of Penetration given by one of its passives so you won't need to wear the 1x Slimecraw in Light weight and instead use a Medium 1x Slimecraw at least in non-solo context.
After some testing on both the raid dummies and in content (also based on previous testing with a different iteration of Oakensoul), it doesn't seem the Berserker enchant (Weapon/Spell Damage increase) will outperform either Poison or Flame Damage enchantments on the Lightning Staff.
Jewelry enchantments can be Weapon Damage glyphs if you have a Dragonknight in your group that can provide the Minor Brutality buff. Other than that, it can be either Weapon or Spell Damage.
An alternative body setup can be 5x Noble Duelist's Silk or 5x Order's Wrath instead of Storm Master for solo instances/arenas or 4 man content!
Skills Review
The Nightblade can be quite single target focused as seen with the emphasis on casting Debilitate, Barbed Trap and letting a Merciless Resolve bow proc go off. Furthermore the Concealed Weapon skill does get buffed by the Major Expedition duration of the Twisting Path skill for higher damage on target.
Single Target Rotation
The opening burst consists of casting Merciless Resolve before combat starts along with Barbed Trap on target then immediately opening with a Soul Harvest ultimate.
After, you should heavy attack weave into a Twisting Path followed by Debilitate then cast Concealed Weapon every time you can.
Since each successful Heavy Attack gives you TWO stacks of Merciless Resolve (unlike Light Attacking which gives you ONE stack per Light Attack), the bow proc will come up sooner than you realize and it's important you don't heavy attack weave on your 5th bow proc stack as that's when you should be releasing it on target.
Ideally you still want to make sure all or most of your DoTs are running if possible when you decide to cast Soul Harvest followed immediately by a Merciless Resolve bow proc due to Soul Harvest's debuff on target that makes it take more damage!
Light Attack weaving only Impale below the 25% execute threshold was tested but sustain wasn't enough. While it was a little more possible with parse food (Max Magicka, Magicka Recovery), it was still rough and the DPS gain due to the sets used wasn't worth it.
Alternatives
After testing in content within the solo context (veteran dungeon), it doesn't seem like the Sap Essence skill or Dark Cloak is strong enough nor sustainable for healing. Instead it can be easily resolved by Resolving Vigor.
Debilitate can be swapped out for Elemental Susceptibility for extra Armor Penetration.
Barbed Trap can be a little risky to use especially if you're depending on Resolving Vigor for healing while also Roll Dodging mechanics or heavy attacks from adds or bosses. A different damage over time skill such as Unstable Wall of Elements.
Instead of Soul Harvest, you can also use Shooting Star or Soul Tether if you need healing while you weave your heavy attacks.
Craft
Warfare
Fitness
Champion Points Review
For Warfare slottables, the Nightblade shares the same ones with the Warden.
If not using a shield with Bastion - which amplifies the effect of casted shields - you can go with Expert Evasion or other slottables from the Fitness tree if need be.
Introduction
This build was used to solo Falkreath Hard Mode. However it can also be used in other content whether that be Trials or solo arenas with great effect!
Recommended race: Altmer (High Elf) or Dunmer (Dark Elf); for solo or 4-man content you can go Khajiit.
If you want to go with 64 points into Stamina instead with a different race like Orc or still even with Dunmer you can, at that point it just depends on the usage of your resource more so than the damage of your resource such as casting utility skills based on Stamina, casting Stamina based healing like Resolving Vigor or performing Roll Dodges and more Blocks.
Recommended food will be a bi-stat Health and Magicka food. For parsing or for fights where you don't need that extra health, you can use purely single-stat Max Magicka food! For soloing veteran dungeons, you may need to invest into a tri-stat food (Max Health, Max Stamina, Magicka)!
Recommended Mundus will be The Thief (for group content like Trials) or The Lover (for solo content and overland).
Potions can be your basic tri-stat restoration potions, however if you'd like to invest further into the build, you could go with Resistance Potions that restore 9206 Health immediately & gives Major Fortitude (30% Health Recovery), grants a unique 5280 Spell and Physical Resistances!
You should have about 25,000 resistances, making you basically a tank with around 30,000 Health (includes Minor Toughness if in group with a Warden).
The potion can be made using the following reagents: Mudcrab Chitin, Mountain Flower, Bugloss