Is this homebrew Mage Bane poison's rarity correct at uncommon?

Question

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 😋 ).

Answer 1

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.