I like the idea of the Lament Barbarian. Feels unique and fresh while something I can see barbarians doing. Doesn't get in the way of your regular attacks either. The level 3 and level 6 abilities are quite good. I don't really think they match up badly against other subclasses either at first glance. Level 14 feels pretty good at first glance - limited use, but two good immunities, extra damage and extra healing is always helpful.
Level 10... feels bad to me. At level 10, we're looking at an average of 19.5 damage from the scream and the cut off for insta-death is 20, so its going to be just a question of a few HP when you're fighting enemies in the 150-250 hp range at this point. Good luck measuring when it'd be worthwhile to use it; frankly, the ability will slow the game down more than help it. Possession immunity is a ribbon. The only real benefit is necrotic and cold resistance. Which... is okay. Cold damage is fairly rare, but necrotic is pretty common.
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Warrior of Venom is just... bad. First, I want to say that I do like the idea behind the subclass - a poison cultivator is actually something that I think is pretty nifty. The theme itself is top tier. But the mechanics just pain me.
At level 3, spending 1 ki point to envenom a -weapon- is a bad trade. Not even your fists, but specifically a weapon is bad. It also doesn't help that you don't have a lot of ki to waste, and its not a damage increase from just spending that 1 ki to Flurry of Blows instead. Probably worse in some cases, because Tavern Brawler is a common Origin Feat for monks, and you're getting a Poisoner's Kit that you can use to coat your weapon in toxin anyways. It'll be a lot better late game when you have ki to spare.
I do want to state that the idea of turning poison damage to acid is a brilliant way to avoid the poison immunity problem, and I think retroactivley applying it to the Poisoner feat is a good move.
Level 6... give up two attacks to make a Magic Action to give someone Poisoned Condition and a bit extra. This one isn't avoiding the immunity problem and, well... Truth Serum is, well, interesting I guess. Charmed is pretty useless in combat as well. Two out of combat social options. The only one I'd use is the one that puts them to sleep. Oddly, the Unconscious condition will end when shaken by an ally, but no word on damage in this case. Which meanst that its either an oversight, or this is a Save or Die effect. I'm guessing the former, which would mean its a worse Stunning Blow. Level 17's Hallucinagenic Breath isn't terrible, but its running up against that poison immunity problem again. Trade one attack for 3d12 poison damage and Frightened is okay. Not exciting, and not something I want to see at level 17. Frankly, this is the kind of thing I'd expect at level 6.
Toxin Refiner/Blood at 11 is actually pretty cool. Sometimes a bit more damage, immunity to poison, and you heal from drinking poisons. Overall a pretty decent suite of abilities, and the only level I think is actually really interesting in the entire subclass.
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Primordial Patron... Okay, its the 5.5e twist of the Genie Patron, which was a 5e twist on the 4e Elemental Pact. I admit, I enjoyed the Pact of the Talisman as well. Gave a whole "you live in a genie lamp" vibe that i thought was pretty cool. I miss it. Anyways, onto the actual subclass.
Level 3 gives you a tiny, tiny AoE. On the one hand, its movable with a bonus action. On the other, it takes a Magic Action to cast. You'll be using this instead of Hex, since you're probably going to be able to hit several monster on the field with tihs every turn, followed by whatever else you usually do with your action or spell slots. Probably going to get very sick of the calls for Dex saves, though. Feels like the kind of thing that will massively slow down the game.
At level 6, however? Your elemental thingy turns from an attack option to a shield - you're supposed to stand in it. Once again, the developers want to just have +CHA as an option, ignoring potential AC shinanigans. Why do they keep doing this when they know its going to be bad? This just screams "dip Fighter, wear the armor, be untouchable." Does anyone thing that 26 AC without spells/magic items is an okay build?
This just feels like they're trying too hard. Like, if they want you to stand in the AoE... just make it an Emination? Granted, then you can't teleport in/around it, but I'm not opposed to getting something a bit different. Couple that with the Invocations meant to provide you with an off-brand Fire Shield effect...
Level 11 increases the size of the AoE, which I'm not sure is a great thing. 20 foot diameter means that there's going to be a lot of friendly fire going on.
Level 14 Vortex feels off to me. A bit of a pull effect when casting a spell, okay, sure. The hour long duration isn't as big of a deal, since its probably going to get in your party's way when dungeon delving. Planar Ally that is REQUIRING you to use your Patron's name as a summon? And your suggested patrons include the Elemental Evil Princes? CR 22 critters? And they show up?! Is this crazy to anyone else?!
