Pobb: https://pobb.in/u98cgfvPa2r1PoB Explanation: Since PoB cannot calculate the 300% increased damage from poison on non-poison enemies, we simulate it in configuration by multiplying the amount of low tolerance we have. Doom is set to 40 since we use Doomsday and Vuln will always have max doom for Hexblast.

I got this build from TheDeathBeam in PoE Discord saying it's league-startable since the build uses many uniques. Played it as a second character though. Tried to mess around with it and achieved somewhat of a good allrounder. Mapping is smooth even as a 2 button build since we don't need full doom to Hexblast the whole pack. Deceptively tanky even at 3.6k HP.Build Concept: Alchemist's Mark takes the strongest poison and put it as a caustic ground beneath the enemy. Hexblast is a really strong 1 hit skill with all the damage added from doom.

Low Tolerance is a node on a medium chaos DoT jewel that says Poison we inflict on non-poison enemy deals 300% damage. We stack lots of Low Tolerance (and poison mastery) and achieve around 1200~2100% increased damage up to how many Low Tolerance we can take. We also use Curse Effect small clusters to solve our attributes and buff our Alchemist's Mark through Hound's Mark. This makes our Alchemist's Mark extremely potent.

We use buffed uniques such as Apep's Rage, Doedre's Scorn, Windscream to play with curses and poisons. They are all 1c uniques as well as BiS for us so corrupted ones with good implicits aren't expensive as well after you farmed currency. Vixen's is there to add +1 curse and apply curses + Alch's Mark with ease. Just keep on collecting currency and either buying or crafting more and more Low Tolerance + Flow of Life (or Brush with Death and 1 Septic Spells to achieve easy 100% poison chance).

Pros:

1. Build deletes packs of enemies with ease. Profane Bloom takes all the credit for map clearing and with Alchs Mark, even if the first explosion from Hexblast fails to kill any, the caustic ground left by Alch's Mark kills anything left. It's potent enough to even melt normal version bosses (Downed all bosses non-Uber version, I don't plan to anyways, I don't kill Maven in most leagues even but this build helps me do it with ease). I once killed Exarch before even the first ball phase. I think the max version with reasonable investments can obtain around 35m DPS on Pinnacle bosses when Deathbeam theorized it. I'm doing really well with my 8m DPS though and I'm satisfied with that.
2. For defenses, the build is deceptively tanky with the use of Divine Flesh and Doppelganger Guise. Doppel is cheap for a unique 6L Armor in comparison to meta body armors. This defense is scalable with the use or Megalovaniac (Born of Chaos + Low Tolerance) but all that's on the market is gone by now (I've snagged one before it was gone). My shield was bought for 1 divine with the max res base and I spammed harvest reforge chaos until I was satisfied, bringing it to a total of 85% chaos res now. Blasphemy + Temp Chains aura is extremely strong, slowing most enemies to a halt and even if I mistakenly lightning warp into a pack, they just don't attack me and I blast them before they can. Forbidden Taste is an oh shit "Use when you take a Savage Hit" that procs frequently since Alch's mark also restore flask use.
3. We use Lightning warp alongside just walking to move around so we always have withering step cooldown. This is just my preference, you can use other movement methods.

Cons:

1. Build is extremely starved of attributes. This can be solved early with a single Astramentis but you'll want to eventually transition off through the usage of attributes on Cluster Jewels instead. It's also a two button build which most will hate, but to me, this is good for my hands since I don't need to hold down buttons (it hurts my hand playing with fast attacking skills).
2. Build can scale to the moon, but investments are extremely expensive due to the usage of Voices, Large Cluster Jewel. I don't know who made what build that uses it but its prices have skyrockted a lot in a few days that I was unable to snag a 3 nothing one to upgrade my build and I don't want to pay 34 divs (and since i'll need 2 to make things much better, that's 68). The other upgrade that can be made is through Malevolence Watcher's Eye but that's expensive as well. Don't need those kind of damages for my playstyle though.
3. Build is slow to scale unless you got money, you'll hit certain walls real quick. Each Low Tolerance helps scale that wall, and you'll hit the next, and the next, and the next, until you finally reach its endpoint. Mine is at level 94 and requiring better Voices now. If you are a fast starter that can farm currency easily, I think you'll be able to leaguestart, but if not, you can start with PConc and transition much later if you like to play the build.

Extra clip: https://youtu.be/020buyaCXyQ killing Exarch and getting hit by flaming hot balls.
Extra clip 2: https://youtu.be/Wtle2cHu0jw another Exarch kill, but this he didn't get to the balls phase. It's taken before the above clip but doesn't show how tanky it is so I didn't upload it at first.