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Cloudkill has an interesting property where the area of effect moves on its own. The spell states:

The fog moves 10 feet away from you at the start of each of your turns, rolling along the surface of the ground. The vapors, being heavier than air, sink to the lowest level of the land, even pouring down openings.

However, we do not receive any direct ruling on what occurs when the effect encounters a solid obstacle. I can think of a number of possible rulings:

  1. The effect stops and the radius remains at the point until dispersed or the obstacle is removed

  2. The gas bunches up along the obstacle leaving a line against the wall instead of a circle.

  3. The fog just disperses itself by trying to force its way through, ending the spell.

Are there any RAW answers to what happens when Cloudkill encounters a wall that the gas could not move through?

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Are there any RAW answers to what happens when Cloudkill encounters a wall that the gas could not move through?

No. The limit of explanation is what you have quoted.

'Solid obsticle' is a relative term. Brick walls are solid but they are not gastight but assuming you do mean impervious to the gas, the DM will have to rule on what happens. Your suggestions are all fine and no doubt there are a number of others that could be made.

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    \$\begingroup\$ While it may be the right answer, it doesn't support itself with any evidence (e.g. pointing to where one might find other related rules/rulings but not this one) or present a logical interpretation given the absence of an official ruling. \$\endgroup\$
    – V2Blast
    Commented Jan 4, 2019 at 0:28
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    \$\begingroup\$ @V2Blast It is very hard to provide evidence of absence. I could, I suppose duplicate everything WotC has published but that would be a copyright violation and exceed SE character limit for answers. \$\endgroup\$
    – Dale M
    Commented Jan 4, 2019 at 0:57
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    \$\begingroup\$ You don't need to "duplicate everything WotC has published", just point to where one might expect it to appear and point out that nothing in that section of the rules addresses it. Or, as I'd suggested, explain how you think it should be ruled given the absence of an official ruling. \$\endgroup\$
    – V2Blast
    Commented Jan 4, 2019 at 1:46
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    \$\begingroup\$ Please update this with sufficient citation or it will be removed. Don't let yourself slip back into the habit of mic-drop answers please. \$\endgroup\$
    – doppelgreener
    Commented Jan 11, 2019 at 17:53
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    \$\begingroup\$ That is not site etiquette, it’s a harmful assumption the community once developed. (I.e.: Answers need not be confined to RAW on request if the answerer thinks their answer is better otherwise. We don’t delete answers for challenging the frame. We do delete answers that don’t show their work, like the current version of this.) Besides, explaining how the reader can see it’s not present in the rules in no way requires going beyond the rules anyway. \$\endgroup\$
    – SevenSidedDie
    Commented Jan 11, 2019 at 18:04
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Option 2 is correct

When Cloudkill encounters an obstacle it will simply stop spreading in that direction, bunching up along the wall or flowing around pillars. We know this because the spell contains the following text:

The fog spreads around corners. It lasts for the Duration or until strong wind disperses the fog, ending the spell.

If the fog can spread around corners is can certainly flow along walls and avoid obstacles. Think how fluids or fog behaves in the real world.

Additional if the fog was dispersed or the effect ending the spell would say so. It doesn't, it can only be ended by a strong wind, losing concentration or the duration ending.

For example consider what happens if you cast Cloudkill in a ten foot wide hallway. The correct behavior would be that the cloud fills the hallway and extends down it 20 feet in each direction.

If this cloud then moved down the hallway into a larger room it would spread out to a radius of 20 feet from its centre point. Spreading around corners and obstacles.

A counter example of a spell that does fail if it encounters an obstacle is Call Lightning.

The spell fails if you can't see a point in the air where the storm cloud could appear (for example, if you are in a room that can't accommodate the cloud).

Cloudkill does not contain any similar text and therefore does not have this restriction.

What if it hits a 100 foot wide wall?

Rereading your question I realized this is the situation you are actually asking about. What happens when no part of the cloud can pass through or around an obstacle?

In this case there isn't a clear ruling by RAW. But we can attempt to extrapolate from the information we are given.

My interpretation of this spell is that is has a 20 foot radius from the centre point, and that centre point moves directly away from you by 10 feet on each of your turns. If we take this as the correct interpretation then I would rule as follows.

The centre point of the cloud continues to move through the obstacle. And the cloud reduces in size as the 20 foot radius from this point approaches the wall. Once the centre point has moved 20 feet past the obstacle the cloud will have disappeared.

It is up to the DM if there is a gap large enough for the fog to have continued on the other side. If they rule there was a gap (it doesn't need to be very large) then the spell continues on the other side much as it was before.

If they rule that the wall was too solid to pass through then the spell effectively ends. Though you could maintain concentration on it, it has no effect.

The best RAW rule we have for this comes from the general spellcasting rules.

A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover.

If we take the pure RAW in interpretation of this then the area would disappear as soon as the point of origin passes through the obstacle, thus providing total cover. For me this ruling doesn't make much sense and I would play it as described above.

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  • \$\begingroup\$ @TimSparkles The wording of the spell is "moves 10 feet away from you" so likely yes, moving and teleporting should change the direction RAW. I have always played it that the direction is set at casting time and it continues in that direction. However there is probably an entire separate question in determining the direction from the wording of the spell. FYI I'm going to clean up our comments as I am unlikely to change my answer based on them. \$\endgroup\$
    – linksassin ♦
    Commented Sep 1, 2021 at 23:55
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The general rule for spells is that their effects are limited by obstacles such as walls, windows, etc. This is simpler for instantaneous spells like fireball, but no general exception is made for long duration spells. This spell also does not say that it is an exception to that, therefore, it is stopped by any solid obstacle, thus ending the spell as soon as the entire cloud would have moved past the obstacle.

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    \$\begingroup\$ Are you suggesting that the spell ends prematurely by the cloud encountering a solid obstacle or that the obstacle causes the progression of the spell to cease at that location? If the latter, can you expand your answer to more clearly describe what happens to the cloud on subsequent rounds. \$\endgroup\$
    – Pyrotechnical
    Commented Jan 3, 2019 at 19:37
  • \$\begingroup\$ The former. I'll see if I can make that more clear. \$\endgroup\$
    – Derek Stucki
    Commented Jan 3, 2019 at 19:40

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