What happens if a vial of acid falls from the sky?

Question

There are various methods that a vial of acid could fall from the sky. Perhaps a familiar is carrying it. Perhaps it is carried by Mage Hand. Perhaps an animal has been trained to pick things up. Then, the Mage Hand travels more than 30 feet away from you and disappears. The vial falls 30 feet into a square dealing 3d6 falling damage to the vial, breaking it. Or the familiar or animal is shot and killed, again causing the vial to fall and break.

Would this falling item be capable of damaging a foe? There is no "originator" anymore to be making an attack roll. Unless the vial itself is making an attack roll...

This is related to this topic: Can Mage Hand pour out a vial of acid?

Answer 1

Exactly that happens — a vial of acid falls from the sky

A DM does not need game mechanics to describe the environment. When a vial of acid falls from the sky, the DM says "A vial of acid falls from the sky. Boom! It shatters to hundreds of glass pieces, splattering the surroundings with acid". That's all. You don't need rules nor rulings just for describing things.

Questions arise when we need adjudication. For instance, the vial of acid is a part of a trap, which a PC has triggered. Players don't want their character to be splattered with acid, so they expect some kind of roll or any kind of countermeasure, in order to feel the situation was fair:

«You hear a subtle "click", it might be a trap! Roll a Dexterity saving throw... Success! A vial of acid falls from the chimney up above. Because of the height, it takes a couple of seconds, so you've managed to jump out right before it hits the ground.»

The DMG explicitly describes dice as a DM's tool, which she can use, but doesn't have to:

Dice are neutral arbiters. They can determine the outcome of an action without assigning any motivation to the DM and without playing favorites. The extent to which you use them is entirely up to you.

Ignoring the dice

One approach i s to use dice a s rarely a s possible. Some DMs use them only during combat, and determine success or failure as they like in other situations. With this approach, the DM decides whether an action or a plan succeeds or fails based on how well the players make their case, how thorough or creative they are, or other factors.

The 3rd edition of D&D was trying to address every possible case in the rules, but apparently this didn't work well. The 5e returns to the "rulings over rules" principle from the 2nd edition.

The rules don't describe everything. It's just a toolset, which a DM can (but not have to) use for making adjudication. The DMG explicitly expects this from the DM (Part 3, Master of Rules):

The rules don’t account for every possible situation that might arise during a typical D&D session. For example, a player might want his or her character to hurl a brazier full of hot coals into a monster’s face. How you determine the outcome of this action is up to you

Tasha's Cauldron of Everything stresses this once more (see page 4, "The DM Adjudicates the Rules"):

[...] the rules can't cover everything. When you encounter something that the rules don't cover or if you're unsure how to interpret a rule, the DM decides how to proceed

So how do we figure out the outcome?

5e is not a physics simulator. Playing D&D is "an exercise in collaborative creation", as the PHB says. "A player triggers a trap with an acid vial attached" and "a vial falls from the sky somewhere" are two very different situations, which require different adjudications. The method of determining the outcome depends on what the situation is:

• If it just happens somewhere, simply describe it to players. Don't waste everybody's time on game mechanics when nobody needs them. If it hurts a NPC, just deal damage and move on. The game designers also suggest to "go with what's best for your story", if you're hesitant about the particular outcome.
• If it happens with players' characters and can potentially harm them, let the dice do their thing. Don't say "you take 30 acid damage", give them a saving throw. Basically you can use rules for traps, or just make it on the fly. A DM shouldn't browse the rule books in the process of the game.
• If it's a PC who drops the acid, but this works only in this particular isolated case, act in player's favor. In this case, you reward creativity. For instance, there is a fight in a lab, and a PC shoves some vials from the upper floor's shelves. The villain takes damage, but the shelves are empty now and the vials are shattered, so that trick can't be repeated anymore.
• If it can be easily reproduced, allow it but don't make it more effective than "traditional" means of fighting. For instance, a player asks their familiar to take the vial and drop it from 30ft on an enemy. If you says it makes more damage and hits automatically, the player would probably do this every time in every fight possible. This hurts creativity and contributes nothing to the story, such things shouldn't be encouraged.

Answer 2

When there is intent to attack by an entity capable of making at attack, you use that creature's attack. If the entity cannot attack (standard find familiar familiars, mage hand), then it can't attack.

Aside from that, there are no rules for this sort of thing, and it's going to be up to the DM; the rules don't allow it, but there's tons of things the rules don't fully cover, and that's why the DM exists.

In your edge cases like "entity carrying vial killed by another entity (capable of making attacks) with intent to harm those below" I might allow an attack (almost certainly with disadvantage) by the creature that killed the entity to cover them timing and aiming the attack to cause the vial to fall on their "real" target.

But if there's no intent, and nothing that could conceivably be making an attack, then it's pure DM whim; this is a mild version of "rocks fall, everybody dies" ("acid falls, maybe someone gets splashed"). There's no rules. Perhaps you determine the square it would land in, and require the occupant of that square to make a low DC Dex save (probably with advantage to account for lack of guiding intelligence in the attack). These are heroes, they rarely get killed or injured by random chance.

Answer 3

According to the Rules As Written, the only way to do damage with a vial of acid is to spend an action to make an attack roll with it.

If you do something else, like dropping it out of the sky, it's possible your DM might choose to rule that it does damage, but otherwise it doesn't.

If your DM rules that this sort of thing does damage, your DM should be careful about what happens when you collect lots of vials of acid at once. We can imagine someone pouring fifty vials of acid into a larger container and dropping that out of the sky on the Big Bad Evil Guy. Would it deal 50*(2d6) damage and kill him instantly? Most DMs would probably prefer for that not to happen, so they should be careful what house rules they issue about dropping acid out of the sky.