[3.19] Build idea: Big Poison with Alchemist's Mark

It's pretty simple actually: abuse the Poisons you inflict on non-poisoned Enemies deal 300% increased Damage.
You can get one from Poison mastery on the tree and more from Low Tolerance notable on Chaos Dot medium Cluster Jewels. So realistically, you can get either 3 (2 medium clusters) or 5 (4 medium clusters) for a total of 900% or 1500% increased damage with Poison.

Now, for it to work, the next poison should be applied only once the previous one is expired, so it should be a slow-hitting build.
Hexblast playstyle fits perfectly since we need to wait for Doomsday anyway.
3.19 gives us a perfect Mark skill for the occasion: Alchemist's Mark and we already have Vixen's Entrapment to autocast it whenever we cast Despair.
Short story: it gives 120% more damage with Poison
Long story: it creates a ground effect that deals Chaos DPS equal to 80% of the strongest poison. And the only way to scale the damage is with the Curse effect, synergizing perfectly with Hexblast passives.
(I left it at 100% more damage with Poison to account for last-minute changes and to be on the conservative side in general)

3.19 has some gear as well.
1) The 3.19 Apep's Rage, we can finally "concentrate" the poison duration to about 1.15-1.25 seconds, which aligns just right with Curse's cast time + Doomsday.
2) With 3.19 The Covenant we can have a 7-link. And the 3.19 Doedre's Scorn adds another 200 base Chaos Damage.

https://pastebin.com/navTv1JP

I went for just one Cluster and 5 Wither stacks to be on the conservative side when evaluating the concept.
I also put 10% less Damage with Poison to account for a small delay between poisons and non-perfect play.
So yeah, all in all, pretty gimped from PoB Deeps warrior's point of view.

Tree:
Click it!

Ascendancy - Occultist:
1) Malediction
Extra Curse, better curses, Malediction
2) Profane Bloom
Hexproof
3) Void Beacon
Just some damage
4) Withering Presence
Damage, utility/QoL for Wither, massive Chaos res

1) Body (The Covenant) B-G-G-G-G-B:
- Hexblast (boom bada boom)
-- Chance to Poison (we want 100%, can be replaced with Curse Mastery + Septic Spells from the second cluster)
-- Deadly Aliments (just DPS)
-- Swift Affliction (DPS, less duration actually helps to sync with Despair-Doomsday-Hexblast cadence)
-- Void Manipulation (just DPS)
-- Efficacy (just DPS, Skill Duration has no effect on Poison Duration)

2) Main Hand (Apep's Rage for big bosses, Bino's Kitchen Knife for clear) B-R-R:
-Malevolence (DPS aura)
-- Divine Blessing (we handle the cost with EB)
-- Increased Duration (self-explanatory)

3) Offhand (shield with defence) B-R-R:
- Despair (manual curse with Doom)
- Purity of Elements (I don't like elemental ailments on me)
- Flame Dash (mobility)

3) Helmet (Doedre's Scorn for big flat damage) G-B-B-B:
- Enfeeble (Defensive curse #1)
- Temporal Chains (Defensive curse #2)
-- Blasphemy (I feel safer with defensive curses always online)
-- Enlighten (LVL 3 nails the reservation, use Conqueror's Potency or small passive in Influence cluster if you don't have the gem)

4) Gloves (Vixen's Entrapment for Extra Curse and Mark trigger) G-R-B-?:
- Alchemist's Mark (uber DPS)
-- Empower (a tiny bit more DPS)
-- Increased Area of Effect (bigger ground effect for better consistency on mobile bosses)
- (dunno what to put here)

5) Boots (Rare with Movement Speed, Life, ES and so on) B-R-R-B:
- Wither (we want 15 stacks asap on bosses)
-- Spell Totem (who wants to self-cast it?)
-- Multiple Totems (stacks aSaP)
-- Faster Casting (yeah, ASAP)

6) Amulet (Rare with Dot Multi, life, and gem levels):
7) Ring 1 (Rare with Life, Res, and stats):
8) Ring 2 (Rare with Life, Res, and stats):
9) Belt (Rare with Life, Res and stats):