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We encountered a homebrew poison, which can coat weapons or piercing ammunition, that is called Mage Bane. Its properties1 are as follows:

Mage Bane

Potion, uncommon

A concentrated, dark green liquid found in earthenware vials, one portion of this toxin is sufficient to coat one melee weapon or three pieces of ammunition. This toxin is considered a poison as regards resistances and immunities.

If a coated weapon or piece of ammunition hits the target, the target must make a DC 13 Constitution saving throw. On a success, the target is unaffected.2 On a failed save, the target takes 1d6 poison damage per spell slot level when they cast a spell (1d6 poison damage for a cantrip) or uses a feature that consumes a spell slot within the next minute.

If this potion is ingesting by eating or drinking it, it takes effect automatically without allowing a saving throw.

Reverse-engineering this potion to replicate it requires proficiency with an herbalism kit and a successful DC 25 Arcana check, and two weeks dedicated to the attempt at making a dose of the potion.

Is the rarity right?

As I looked at some other magic items, such as a wand of web and its DC 15 save, I think the rarity is correct as regards the save DC. But I'd like some more eyes on this before I consider introducing this in play as a DM.

Does this consumable magic item need any modification to remain within balance? Or should its rarity be bumped up to rare?

1 One of our party members attempted to reverse-engineer this and succeeded.
2 Note that there is no opportunity to roll a saving throw after the failed save.


Clarifications from comments:

  • The DM who designed this item (@BenjaminTHall) has commented that it is intended to counter the use of magic, so any use of a spell slot, including a Paladin or Warlock smite that is spell-slot powered, would also trigger the effect.
  • @Molot asked how many hits this is good for on a melee weapon. Right now, "one" is the answer. I had considered allowing for a second hit to attempt to apply it, and offer advantage to that save, but it got too fiddly for me at that point.
  • A question from @doppelgreener: If I cast a level 1 spell with a level 9 slot, how many dice damage do I take? The answer is 9d6.
  • @SeriousBri asked: what does reverse engineer mean in game terms? It allowed us to create single-use doses of it, so just what it means in normal English usage - try to replicate something by taking it as you find it and making a copy of it without actually knowing the specific formula, or the specification, or the drawing, or the recipe by which it was originally created. (And good luck reverse engineering my two-time-award-winning chili recipe, for example 😋 ).
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  • \$\begingroup\$ I assume by asking straight for rarity that you are suggesting it is already balanced? Because finding rarity (itself almost meaningless) on an unbalanced item (which I personally think this is broken beyond belief) is tricky. And does reverse engineering give a formula that can be used to craft the poison, or does it let you actually craft the poison? Also, thinking about rarity, I don't believe it is intended to be a measure of power, but a measure of actual rarity, so how common is this in your world? \$\endgroup\$
    – SeriousBri
    Dec 6 '21 at 14:06
  • \$\begingroup\$ @SeriousBri Rarity is related to power considerably; look at the progression in the DMG tables. \$\endgroup\$ Dec 6 '21 at 18:26
  • \$\begingroup\$ @SeeriousBri And does reverse engineering give a formula that can be used to craft the poison, or does it let you actually craft the poison? It lets you make it successfully. In our game, it resulted in the paladin making a potion very much like it with slightly different effects. I assume by asking straight for rarity that you are suggesting it is already balanced? No, I am not. Why do you assume that? Why do you think I bothered to post a homebrew review? I think it is broken beyond belief Well, I encountered it in play, and I cannot agree with your opinion. But it was nasty. \$\endgroup\$ Dec 6 '21 at 18:40
  • \$\begingroup\$ @Mołot A core reason for posting this question is to look for munchkinproofing, which is what my concern is at the current rarity estimate of uncommon - not to get wrapped around the axle regarding a common term. One of the questions further up asked "how common is this in the game world" and that's a very good question, and one that I do not have an answer for yet; there are certain parts of my world were wizards are still hunted, and other places where they are tolerated. Rare may be a better classification, or, region/faction unique, like drow poison and armor in AD&D 1e were. \$\endgroup\$ Dec 7 '21 at 0:49
  • \$\begingroup\$ @Mołot The other option is to specify that it cannot be replicated without the formula, nor by use of a magical spell short of wish, so that the ability to create it is gated by the DM in terms of when or where a forumula may or may not be available, in those cases where a group does have the downtime to craft. But that get's well beyond item assessment and into DM/Campaign issues that vary so much it's hard to pin down. \$\endgroup\$ Dec 7 '21 at 1:21
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The only uncommon potion with a negative effect is the Potion of Poison, so let's compare the two:

Potion of Poison Mage Bane
DC 13 Con save DC 13 Con save
Disguised as beneficial potion Can coat weapons (which means you can be hit with it multiple times in one turn)
Guaranteed 3d6 poison damage Unless it's ingested, there's no effect if Con save is successful
3d6 poison damage per turn for up to an hour 1d6 poison damage per spell whenever you cast a spell within the next minute

Which of these is the more powerful potion is subjective, but IMO it would be a good idea to have them at the same level of rarity; uncommon is probably the best choice.

As a side note, it seems unlikely that you could convince a wizard to drink a dark green liquid and then start casting spells within the next minute. Perhaps instead of bypassing a Con save when ingested, it should have increased duration.

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  • \$\begingroup\$ Your last suggestion is a good one, and ingestion is not an expected means of being under the effect - that is put in there to cover the odd eventuality of ingestion (like a large or huge monster that can cast spells ingesting it somehow during a battle) ... or someone accidentally drinking it... \$\endgroup\$ Dec 21 '21 at 13:08

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