[2.4] Vortex Necromancer using Decay and The Scourge

Apologies in advance for the long load times and poor quality, making do with what I have :)

Deathless Atziri run.
For dual Oversoul and Atziri switch out increased AoE for Concentrated Effect. I keep increased AoE in for Trio as I like to keep my distance from them.
https://www.youtube.com/watch?v=bJ66st0mq6M

Tier 9 Promenade with Elemental Reflect and Cold Res.
Demonstrating clear speed with a Lv 1 Vortex, essentially clearing with Decay alone. Damage will be higher with a levelled Vortex.
https://www.youtube.com/watch?v=TmRYiqjgY8Y

Tier 10 Courtyard Twinned Boss Kill.
https://www.youtube.com/watch?v=qGDaOkSu1VQ

Can do all map mods - some combos like No Regen and Hexproof are annoying but doable.
Has done Atziri and Uber Lab deathless.
Reasonably cheap to get going - Essence of Delirium and The Scourge are the only mandatory items. Carcass Jack and better rare items will be more expensive but not needed initially.
Incredibly fast, fun and active to play - the build uses a high AoE Vortex, Wither, Flesh Offering and Whirling Blades as its active skills.
Safe boss kills - tag once, run away and let Decay do its thing. Reapply as necessary. Chill, wither and temporal chains slows bosses to a crawl.
Black Hole Vortex looks AMAZING.

Potentially laggy - depending on your computer the Vortex effect may cause a bit of lag, especially if stacked.
Unknown DPS - Decay DoT isn't listed on the tooltip so you won't know how much DPS you are doing. It's best to go by feel.
Not as tanky - compared to typical Vaal Pact, CI, high armour or Pathfinder builds.

The Scourge: Makes it so "minions deal X damage" mods can be treated as general "increased damage" mods for you. You get 70% damage just by hitting enemies and an additional 175% through passives/Necromancer nodes/Bone Helm. I found this to be the best option for scaling damage as it boosts the initial hit of Vortex, the DoT component and the Decay effect.

Decay: Crafted on a weapon using Essence of Delirium. This thing is amazing, does more damage than a Lv 22 Essence Drain and is nearly on par with a Lv 26 Caustic Arrow. From my testing it appears that the Decay DoT scales with the support gems of the skill used to inflict it, as well as any relevant passives on your skill tree. Even though it looks like a poison effect it isn't counted as one - tested using Bino's (did not spread poison nor did I get life recovery on kill.)

Vortex: Great skill to use with Decay for two reasons: 1) Has large AoE with passives, gem and Carcass Jack, and 2) The "Modifiers to Spell Damage apply to this skill's Damage Over Time" scales the damage of Decay. Essence Drain would be another great skill to use.

With all four Ascendancy nodes you get 160% damage, 53% cast speed (8% from travel nodes, 5% for running Clarity, 10% for consuming at least 5 corpses, 30% from Flesh Offering) and 30% Skill Effect Duration which helps Flesh Offering, Wither, Vortex and Immortal Call.

Using Flesh Offering also provides 30% attack speed which helps Whirling Blades and an additional 25% movement speed. You also get 20% Elemental Resistance and 21% Chaos Res which helps with gearing.

Vortex - Controlled Destruction - Rapid Decay - Void Manipulation - Increased Area of Effect - Concentrated Effect
Whirling Blades - Faster Attacks - Fortify (when used in the Scourge FA and Fortify will also apply to your Spectral Wolves).
Wither - Faster Casting - Spell Totem
Flesh Offering - Increased Duration - Clarity - Summon Stone Golem (Clarity is kept at Lv 1 as we just need it for the Commander of Darkness bonus)
Cast When Damage Taken - Immortal Call - Increased Duration - (toss up between Vortex, Frost Bomb and Bone Offering, play around with which one you like best).
Vulnerability - Temporal Chains - Warlord's Mark - Blasphemy (you won't run all three, but can swap between Vulnerability and Warlord's Mark depending on map difficulty and mods.

Can't really comment on levelling with this character as I levelled from 1-60 using the Essence of Hysteria glove enchant (which has since been nerfed to require lv 50). The upside to this change is Decay now has a lv 50 requirement (instead of lv 78) so we can transition to it a lot sooner.

Order doesn't matter that much, I left Beacon of Corruption last due to less total damage/utility when compared to the other three.

My order: Mistress of Sacrifice - Commander of Darkness - Spirit Eater - Beacon of Corruption

The Scourge:
Rolls don't really matter as the 70% damage and flavour mod are always the same. Could look for a 1.7+aps one to make your Whirling Blades faster.


Delirium Weapon:
A 40% ele damage Sceptre will be better than a dagger due to being able to roll cast speed. We also aren't a crit build so the implicit is wasted. Get a Void or Opal Sceptre with an ilvl of at least 84 and aim for a high spell damage or cast speed roll when using Essence of Delirium. Master craft whatever you are missing.


Helmet:
The new bone helmet is great as it gives 40% increased damage - it's basically the Uber Lab Vortex enchantment but without the RNG! Make sure you don't use a Lab enchantment on it as this will remove the minion damage implicit.


Body Armour - Carcass Jack:
Not entirely needed but pretty useful due to life, res, aoe and damage. I may play around with a Lightning Coil for more defense but getting 3-4 Blue sockets on it would be a nightmare.


Ring - Doedre's Damning
For +1 Curse. Saves 6-7 points if we were to get it from Whispers of Doom. If you're lucky enough to get a +1 curse corruption on an amulet you can drop this and use a life/res ring instead.


Rest of your gear should be focused on getting as much life and resistance as possible.

My current flasks are lazily rolled, just aimed for the typical anti-bleed/freeze mods. Still need to sort out a Curse Immunity flask.

Witchfire Brew is nice for added defense and increased DoT. Also helpful in Blood Magic maps when we can't run Blasphemy auras.

Normal: Help Oak
Cruel: Kill All
Merciless: Kill All (Kraityn is a good choice if you are generating Frenzy Charges, Oak would also be helpful due to our Warlord's Mark setup.)

The build uses 7 jewel slots for a total of 250-290% increased damage.

Mods to look for in order of priority:
Spell Damage
Spell Damage while Dual Wielding
Minions deal increased damage
Area Damage
Damage Over Time
Damage
Chaos Damage (not that great as it only benefits Decay and not Vortex)

What do you use for defense?
Defense consists of Spectral Wolves aggro, Temporal Chains, 3 Endurance Charges, Fortify, Immortal Call and chill effect.

You also get about 180% life on the tree which at my level and decent gear gives me 5200 life. At lv 90 I hope to be pushing 5600 life or more.

Any other usable Ascendancies?
Trickster and Occultist would be other worthwhile alternatives due to their Damage Over Time and Chaos nodes. However to get the most out of your nodes I'd probably drop the Scourge and Vortex to use Essence Drain in a +1 Delirium Bow with Soul Strike instead.

No idea which choice is the best, Necromancer just sounded a lot more fun.

Why don't you pick up the Corruption node?
It would take 5 points to get a total of 50% increased damage, and only benefit the Vortex DoT and Decay DoT. I prefer to spend the points on jewel sockets as they only take 3 points to get and I'm able to get 35%+ damage on them. TLDR it's more efficient to get jewel sockets.

Dealing with Elemental Reflect?
Swap out your levelled Vortex gem with a Lv 1 Vortex gem. You will lose a little bit of damage but can still clear easily with just Decay.

Less/No Regen Maps?
Running Warlord's Mark on blasphemy is enough to leech back mana. The issue is when the map also has Hexproof in which case I would turn off Blasphemy auras and use a mana flask.

Blood Magic Maps?
Turn off blasphemy auras. With 3.9% life regen on the tree you have enough to sustain skill usage. I would still run Clarity as it only uses 34 life and grants the damage and cast/attack speed bonuses from Commander of Darkness.