If I drop a bag containing 15 flasks of alchemist's fire on a giant and it hits, will it take 15d4 fire damage and so on?

Question

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?

Answer 1

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.

Answer 2

Curse of Strahd features a trap triggered by one vial of alchemist’s fire that drops and detonates 100 more vials in a 10-foot by 20-foot wagon and results in an explosion that deals 10d10 fire damage to everyone with 30 feet of the wagon who fails a DC 12 Constitution saving throw, or half as much as to those who pass. Everyone inside the wagon or within 5 feet of it has disadvantage on the saving throw.

Answer 3

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.

Answer 4

The damage would not stack and neither should the duration overlap. Read on for further details...

The damage would not stack and neither should the duration overlap. The reason being is that the affected target or location is set aflame and that flame is caused by the same material and the same effect. Also common sense tells me nobody who is on fire is going to be like, "oh let me snuff this little flame, then this little flame" and so on. They are going to snuff out the flames entirely. I would even go so far as to say alchemists' fire should have a limited duration. The reason being that it is not magical and the RAW used the term "ignite" which is synonymous with "combust" which means to consume by fire. Thus the material is consumed by the flames and consume means to use up. If the material is being used up then it wouldn't last forever, unless the material is also infinite in supply.

I hope this helps, take care.

Below I have provided some further clarification on some of the rules on the interaction in question:

The ranged attack is not strictly necessary for the effect to take place. The only thing is that is truly necessary for the effect to take place, based on RAW, is that the material inside the vial is exposed to air. So it could be thrown, dropped, poured, spilled, or even just opened (if the liquid is only kept from the air by a stopper, cork, etc.) So long as the atmosphere of the location at the time of exposure contains "air" the effect will take place. The reason that the items description states that an ranged attack can be made (as if using an improvised weapon), is so that there is a method for which to use the item in combat without needing DM approval.

RAW includes specifications for falling damage and object hitpoints. How far the flask needs to fall is dependent on what kind of material it is made of. It is most likely made of glass, crystal, clay or wood. Either way the HP of the container can be easily calculated as follows:

We know (no assumptions) that the flask is fragile as it is meant to break when thrown. We also know that the flask is tiny in size as it is made to be held in one hand (i.e. it lacks the two handed property) and we know it weighs 1 pound. Page 246-247 of the DMG lists the HP of a tiny, fragile object as 2(1d4) HP. Page 183 of the PHB states that a creature that falls takes 1d6 points of bludgeoning damage per 10 feet that they fell, to a maximum of 20d6.

Here is the ONLY assumption in the calculation and it is really just common sense. The assumption is that colliding into things at high or relatively high velocities is damaging to both living and non-living things. This is an easy write off as if it were not true then a non-magical item like alchemists fire would never work as the whole premise of the item requires an impact to shatter the container and thus expose the contents to air in order to trigger the ignition and begin its effect. If impacting the ground won't break the flask then neither will hitting a creature with it. If such were true then weapon attacks wouldn't work either because they require collisions to be damaging in order to cause harm. Also if such were true then a mountain dropped from the sky (it's an object) onto...anything, would cause harm to neither the mountain nor the object it fell on.

So it can be easily concluded (given the 2HP example listed in the DMG) that a fall of 10 feet has a roughly 33.33% chance of breaking the flask and a fall of 20 feet has a 100% chance of breaking the flask.

ADDITIONAL NOTE: It may be abusable as the item is written in the PHB however, I would like to say that even though it may be abusable, it isn't really a good idea to do so. Even though a player may want to abuse alchemist's fire to make their build stronger or get that edge over the enemy, if there is no magic involved in the items' effect then common sense should prevail. Think of it this way if you're playing in a campaign and you abuse alchemist's fire by stacking the damage or durations, or if it is allowed to burn infinitely even though it is finite in quantity, then how would it feel to have the same abuse of alchemist's fire (or similar item/effect) used against you? It would probably seem pretty lame in the moment and make very little sense. Your character would probably die and you would probably feel cheated. Well this is likely how the DM will feel if alchemist's fire (or similar items/effects) are used to cheese their encounters. Remember the DM is also a player at the table, they may have a different role in the game but they are also there to have fun and play the game. A little consideration and common sense will see you through to the best course of action.