What effect (if any) should Fog Cloud have on a Dust Mephit?

Question

A party was recently exploring an old, abandoned library. They triggered the emergence of several Dust Mephits, and a young wizard responded by casting Fog Cloud. As the DM, I was faced with three options:

1. Strict Interpretation - Since water is not listed as a vulnerability for Dust Mephits, the spell has no effect aside from the inconvenience of visual obscurance.

2. Moderate Interpretation - The fog cloud has the obscurance effect, and also suppresses the dust generated from the death burst of the mephits

3. Liberal Interpretation - The fog cloud suppresses the mephits themselves, and they simply cease to be.

Generally speaking, this falls into a set of DM calls that are outside the rules of any of the guides, but should obey the notional laws of physics that the players are comfortable with (or they will have little reference upon which to make creative decisions). Is there any guidance available that ameliorates the Strict interpretation to require that the properties listed for the spell and the creature to align with natural intuition regarding the physical world?

NOTE: What happened in the actual game was a screaming match. I liked the use of the spell (particularly by the young player) and decided to reward him with a liberal interpretation of the rule (based on my many years of living with both dust and water). The other players reacted badly, claiming that I wasn't following the rules. I've never had a party complain that monsters were defeated before...

Answer 1

By RAW, Fog Cloud would have no physical effect on Mephits and would create an area under heavy obscurity

Monster Manual, page 215:

Dust Mephits are imp-like elemental creatures that are a mix of Earth and Air. They are vulnerable to fire and immune to poison. They only have darkvision.

PHB, Page 243:

Fog Cloud creates a 20' radius sphere of fog that puts the area under Heavily Obscured, unless a gust of wind moves it.

Based on these two items, the fog would not have any negative RAW effect on the Mephits except for limiting their sight due to making the area Heavily Obscured.

The MM specifically puts a vulnerability to fire and not water. Even if you were to go with a Rule of Cool ruling, it doesn't really work because they aren't vulnerable to water.

In addition, the Dust Mephit's Blinding Breath may even push the fog out of the way.

Rule Zero

Ultimately, you're the DM, though. If you want to run it to work like the way you did, that is entirely within your rights. They may not like it, but you had an idea, it made sense to you and you went with it. The hard part now is staying consistent. Will you always allow Water effects to disperse mephits? Will you allow water to disperse a fire elemental? Etc etc. The on-the-fly rule of cool isn't necessarily wrong, but you do have to try and remain consistent as well as within the expectations of players. However, if they are metagaming knowledge of mephits and complaining, that's kinda wrong, too.

Answer 2

If it made sense to you, it was the right call. As the DM, you have the right to create the world as you see it and narrate, as well as resolve the players' actions according to your understanding of your world.

But! Be prepared for prepared for shenanigans like Fog Clouds killing Dust Devils and possibly even arguments in the future for killing Air Elementals with it as well. If you made a mechanical improvisation, be prepared to rule the exact same way every time it comes up.

Now that I've cleared up that the DM's ruling is always the right ruling (for your table, at least), this is what really is your problem: your players didn't think the ruling was fair.

In my humble opinion, it wasn't (fair). The players are understandably upset because why does this 1st-level spell get to have a better effect than < insert another spell here >.

Saying yes, but...

Alright, so it made sense to you that the mephits would "cease to be". That's cool, it's a nice change of pace and rewards the player for thinking creatively, the trick is to enforce a mitigating factor that makes the game fair. This is one way I can suggest, which I personally use all the time:

Allow a save. Introduce a risk of failure for a certain action, no matter how creative it is. This way, the creative-thinking can still be rewarded but isn't overly powerful. Whether it is an Ability Check, a contest, or a Saving Throw as I detail here, you allowing the action to have a chance of success is your job as DM, let the dice decide if it succeeds (or by how much it succeeds).

Not all the mephits should have dispersed at the contact of water vapor. Let all the mephits roll a Constitution Saving Throw against the spellcaster's DC. On a fail, the mephits are knocked unconscious for the spell's duration (or longer, if you want).

Answer 3

RAW answer is provided by NautArch, but here's the way I would adjudicate it.

The first question is did the Wizard intend to do harm, or just obscure the area? If the latter, the issue is resolved by RAW, and you don't need to worry about it. If he intended to do harm, along the lines that moisture in the air should harm the mephits made out of dust, then we continue:

I would make the Wizard roll an intelligence check. If he rolls suitably high (you determine the DC, I'd probably say 15+ is a success, with higher rolls doing more), the spell has an effect on the mephits. Simplest case, it reduces the damage of their death burst, best result is it neutralizes the mephits for now (dispersing them or similar, but don't describe them as dying). Make it clear that this is not precedent for future rulings, but worked in this scenario. This sets a few precedents:

1. Lateral thinking is encouraged. This isn't a bad thing you just need to be able to react to it.

2. It may not work next time (because of the ability check)

3. It allows you to encourage the wizard to create a spell that more specifically handles this situation, e.g. a spell called Water Burst that deals damage or banishes earth and fire based elementals or mephits (not sure offhand what their typing is)

This allows your wizard some fun, and rewards quick thinking, but doesn't negatively turn the game's rules into abuse territory.