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While flying over a giant, I drop a bag containing 15 flasks of alchemist's fire and hit them. Will the giant take 15d4 fire damage and so on? Will the damage also be 15d4 for the following round?

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Features of the same name do not stack

This is a rule found in Dungeon Master's Guide and Xanathar's Guide to Everything. Xanathar's version is more elaborate:

Combining Different Effects

Different game effects can affect a target at the same time. For example, two different benefits can give you a bonus to your Armor Class. But when two or more effects have the same proper name, only one of them (the most powerful one if their benefits aren’t identical) applies while the durations of the effects overlap.

(I'll exclude the DMG version since it's mostly identical, but those interested can find it on DnD Beyond here)

A target that is under the effect of ten Alchemist's Fire effects will only take 1d4 damage at the start of their turns as if they were under the effects of only a single Alchemist's Fire.

However, if they manage to extinguish the fire, they would still be under nine Alchemist's Fire effects, then eight, then seven and so on. The upside of using multiple Alchemist Fires is that the target has to technically spend an action to extinguish each of these flames.

Rules-as-written, though, dropping an Alchemist Fire on one's enemy does not work:

As an action, you can throw this flask up to 20 feet, shattering it on impact. Make a ranged attack against a creature or object, treating the alchemist's fire as an improvised weapon.

You need to use a specific action to throw the flask --- note that this also means you cannot use Extra attack to throw multiple flasks (but you can use Action surge).

As a GM, I would rule that the effects of Alchemist Fires do not stack at all (and therefore can be extinguished using a single action no matter how many affect the target) to prevent an exploitable strategy of setting foes on flames they cannot hope to extinguish and retreating to safety while they frustratedly try to succeed in a dozen Dexterity checks before those 1d4's wear out their hit points.

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    \$\begingroup\$ The DMG version does includes the following helpful sentence: "For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again." \$\endgroup\$ – Medix2 Dec 4 '19 at 14:22
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    \$\begingroup\$ "Rules-as-written, though, dropping an Alchemist Fire on one's enemy does not work" RAW also says that it "ignites when exposed to air," so if the fall is sufficient to break the container in which it is held, an attack roll might not be necessary. \$\endgroup\$ – GrandOpener Dec 4 '19 at 20:38
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    \$\begingroup\$ @GrandOpener But isn't that "if the fall is sufficient to break the container" part specifically not covered by RAW? That's reason there's a problem-- the critical step in successfully dropping a sack of Alchemist Fire is not defined in the rules. I'm not saying it's unreasonable, just that it fundamentally isn't RAW. \$\endgroup\$ – Upper_Case Dec 4 '19 at 23:19
  • \$\begingroup\$ @GrandOpener: you need it to break on the target, not glance off and break on the ground, leaving a puddle of sticky incendiary goop on the ground, not the target. That's my understanding of what the attack roll is for, and would apply whether you're dropping or throwing. \$\endgroup\$ – Peter Cordes Dec 5 '19 at 4:11
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    \$\begingroup\$ There are some monsters who use attack rolls for dropping on targets (from the top of my head, Piercer and winged kobold, iirc), so there is precedent for dropping an object onto a target being an attack roll. \$\endgroup\$ – BBeast Dec 9 '19 at 11:03
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You need to make an attack for Alchemist's Fire to activate

As an action, you can throw this flask up to 20 feet, shattering it on impact. Make a ranged attack against a creature or object, treating the alchemist's fire as an improvised weapon.

By the rules you cannot just drop the flask and have it ignite.

The reason for this is not clear, perhaps a certain amount of force is needed to shake up the liquid when throwing it, perhaps if the glass shatters too easily then the reaction will happen too slowly and fizzle out. There are no rules given for igniting the fluid besides throwing it.

A DM could reasonably allow it

It would be reasonable to rule that dropping the flask from a sufficient flask could activate it, perhaps with disadvantage to represent how imprecise dropping the flask is.

Each flask costs 50gp. You can reliably deal as much (if not more) damage, even without proficiency by shooting with a longbow from 600ft.

The fires need to be extinguished one at a time

The text states:

A creature can end this damage by using its action to make a DC 10 Dexterity check to extinguish the flames.

You can only extinguish one fire per turn since you only have one action per turn. This makes hitting a monster with multiple Alchemist's Fires a powerful tactic.

Alchemist's Fire stacks

The text for Alchemist's Fire states:

On a hit, the target takes 1d4 fire damage at the start of each of its turns.

There is no indication that it cannot stack.

However there is an errata to the DMG (which doesn't usually contain rules) which states that "when two or more [spells, class features, feats, racial traits, monster abilities, and magic items] when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap." so in the damage would not be increased, but each fire needs to be extinguished individually still.

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  • \$\begingroup\$ @kviiri The 2018 errata effectively updates the dates so they supersede XGtE which supersedes the 2016 errata which overrules the original rules. I don't have access to XGtE which is why I was asking where you were quoting. I included a note about the original errata (which is also the latest) anyway. \$\endgroup\$ – gszavae Dec 4 '19 at 9:08
  • \$\begingroup\$ When the 2018 core rules errata were released, I think they accidentally overwrote the original 2016 errata PDFs (i.e. the actual URL). The relevant rules section was actually added at least as of the 2015 DMG errata. It says "But when two or more game features have the same name [...]", and then says at the end: "Game features include spells, class features, feats, racial traits, monster abilities, and magic items." It's not stated whether this is an exhaustive list; I'd assume not. \$\endgroup\$ – V2Blast Dec 4 '19 at 10:24
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    \$\begingroup\$ Your last section is a bit confusing. Your title says they stack, then you say say in the body that there's no indication it can't stack, but then you say that the damage doesn't stack. I might be missing something but I think it would be good to update the earlier wording. \$\endgroup\$ – Rubiksmoose Dec 4 '19 at 13:47
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    \$\begingroup\$ "By the rules you cannot just drop the flask and have it ignite." what about other effects, like Catapult spell? There are other ways to get flask broken and content exposed to air. At least one way. And the flask says it ignites when exposed to the air, so it does, right? \$\endgroup\$ – Mołot Dec 4 '19 at 14:51
  • \$\begingroup\$ @Rubiksmoose Are you talking about the heading for the last section? I say that there is an errata to the DMG which indicates that they may not stack depending on your reading and if you are playing with the DMG and if you are playing with this specific errata. And even so they /do/ stack but the damage from only 1 at a time activates. \$\endgroup\$ – gszavae Dec 5 '19 at 0:42

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