One of my players is frustrated with the Assassin subclass. The "Assassinate" ability is powerful
when it works, but it hasn't been very reliable for his character. He doesn't care much for the sneaky spymaster stuff like acting and wearing disguises either, but he does enjoy potions and alchemy. His character recently joined a faction The Iron Cauldron (an underground group of apothecaries, poisoners, and drug dealers who specialize in illegal substances), and he asked about ways to incorporate that into his subclass instead of the "assume a false identity" stuff.
This is what I came up with on the fly; I'm sure it needs to be balanced and reworked a bit....and that's what I'm posting it here for.
What do you think?
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Variant Assassin: Poisoner
POISONER (replaces ASSASSINATE)
You learn the Poison Spray cantrip at 3rd Level. The Save DC for this and all other class features is (8 + your Intelligence modifier + your proficiency bonus).
If your player is happy with this, go for it. I see it as a significant downgrade in power and flavor, but if it's reinforcing their fantasy and they're happy then it's fine.
VENOMED STRIKE (replaces ASSASSINATE)
Starting at 3rd level, your Sneak Attacks deal +1d6 points of poison damage, and the target must make a Constitution save or gain the Poisoned condition for 1 minute. The creature can repeat its save throw at the end of each of its turns, negating the Poisoned condition on a success. This poison damage increases to +2d6 at 5th level, +3d6 at 11th level, and +4d6 at 17th level.
You need to define what the Difficulty of the CON save is.
It's a bit overpowered, but not broken, and it matches the flavor. However, this will increase handling time at the table because (a) the Rogue almost always has Sneak Attack or at least it's supposed to, (b) every Rogue attack now involves a Con save and tracking the Poisoned condition, and (c) this is an ongoing save the GM needs to remember.
If Sneak Attack were less of a "happening every round" ability and more limited, you wouldn't have as much of increased handling time. But I'd be very wary of this implementation, because I personally prefer streamlining 5e's combat as much as possible. YMMV depending on your group's style though.
Also, if their build involves two-weapon fighting or getting Action Surge,
haste, or Extra Attack -- that's when this feature could start to get a little overpowered.
INCREASED POTENCY (replaces INFILTRATION EXPERTISE)
At 9th level, living creatures no longer have immunity or resistance to Poison damage against poisons that you apply yourself, and Antitoxin no longer provides any benefit. (Non-living creatures, such as Constructs and Undead, remain unaffected.)
This one gets a yellow flag from me. I would advise rewriting. I get the reason for doing this – his shtick is poison, so many monsters have poison resistance/immunity. But... I think that's the tradeoff. Also, making a devil or whatever susceptible to poison devalues the narrative established about what that monster is like and how its society functions (or fails to function). Instead, I would lean into there being a CHOICE where the assassin can PREPARE a specific formula for a poison that bypasses a specific type of creature's poison resistance/immunity, and I'd crib the time/gold cost from Infiltration Expertise. That would minimize the mechanical/narrative fallout while also reinforcing what I think your player wants - the methodical poisoner/schemer who has prepared the very poison they need for this foe.
TRANQUILIZER DART (replaces IMPOSTER)
Beginning at 13th level, when you poison a living creature with your sneak attack ability, you can use your Reaction to cast Hold Monster on that creature. (Casting the spell in this manner does not require Concentration.) Once you use this ability, you cannot use it again until you finish a long rest.
Uff. Balance wise you're fine. However, this one is compounding increased handling time of Venomed Strike with
another saving throw. If your group goes in for this "chained series of complex maneuvers and tracking multiple conditions across multiple foes round after round", then my objection probably isn't relevant for you. Personally, I would look at the drow poison which has a "fail by 5+ and unconscious" clause IIRC, and instead go with something like "fail by 5+ and paralyzed while poisoned in this way." That at least consolidates this power into the saving throw made against Venomed Strike & avoids having to reference the Hold Monster spell (less cross-referencing = faster play).
DEADLY TOXIN (replaces DEATH STRIKE)
At 17th level, a living creature Poisoned by your Sneak Attack automatically takes 20 (6d6) points of Poison damage at the start of its turn.
This one requires a bit of numbers comparison.
Death Strike: A 17th level rogue with a rapier does 41 (1d8+5+9d6) when Sneak Attacking a Surprised creature. The damage is doubled to 82. How often do they get to Surprise? Let's say they get this twice a session, on average looking at the long run. That means Death Strike is contributing an additional 41+41 = +82 damage.
What about Deadly Toxin? Sneak Attack triggers most rounds. If we assume two combats, say, of 3 rounds each, that's 6 rounds of combat. Just an estimate matching the "twice a session" guess for Death Strike. Monster saves are usually pretty abysmal, so most of the time a monster will fail its Con save - let's say 2/3 times it fails. Something like this for each combat...
Combat #1 = + 60 damage
Round 1: Monster A fails save, gets Poisoned
Round 2: Monster A takes 20 (6d6) poison damage, fails save again. Monster B fails save, gets Poisoned
Round 3: Monster A takes 20 (6d6) poison damage, succeeds save. Monster B takes 20 (6d6) poison damage, fails save.
Combat #2 = +40 damage
Round 1: Monster A fails save, gets Poisoned
Round 2: Monster A takes 20 (6d6) poison damage, fails save again. Monster B succeeds save.
Round 3: Monster A takes 20 (6d6) poison damage, succeeds save. Monster C fails save.
So you've added approximately +100 damage with Deadly Toxin compared to +82 with Death Strike. Even though poison is more often resisted, you have a feature that works around that (Increased Potency). AND - this is the big one - you've made it MUCH easier to deal this damage (Sneak Attack is far easier to get than Surprise).
I would look for a way to further restrain how often this power can be used – either conditional trigger limitation like Death Strike or limited #/day – to rein it in. Right now it is overpowered (not in terms of finite per instance damage, but in terms of that damage compounding due to how frequently this triggers).
EDIT: One feature that I don't see here yet is how this subclass interacts with
existing poisons in the game – Assassin's Blood, Serpent's Venom, anything harvested from a monster, etc. I think there's room for an interesting feature there that slightly enhances such poisons in this PC's hands, that would really play to the fantasy they're aiming for. Something like "apply poison as a bonus action at 3rd level" & "save DC of applied poisons equals your save DC for Venomed Strike."