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The cloudkill spell says,

The vapors, being heavier than air, sink to the lowest level of the land, even pouring down openings.

Does that mean that a hole on the floor is an effective countermeasure to the spell (as it drains away from your floor), that the spell effect extends downwards if given the chance in a cylinder or some other shape (making it terribly efficient in a tower), or something else entirely?

In any case, what happens in future turns after the hole interaction? (Vague, I know, but even wording the question is contingent on the above's answer).

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The cloudkill spell creates a "20-foot-radius sphere [...] centered on a point you choose within range", spreads around corners, and

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.

So yes, it'll trace along the floor, whatever the floor does, including dropping down into a hole.

My interpretation of this is that the 'pouring down openings' thing is meant to indicate that it doesn't need a 40-foot opening to move around. The sphere doesn't roll across the landscape like a ball, but rather the center-point runs along the surface of the ground, following the terrain, and the fog fills as much of that twenty-foot area as it can.

So if you had the sphere approaching a very deep pit and the center-point ends its movement right on the lip of the pit, you'd have a nearly complete sphere of fog centered on the edge of the pit, then the next turn the center will move 10 feet straight down along the wall of the pit (or freefalling if it's like just a hole in the middle of an unsupported floor) and continue to fall at 10 feet per turn until it reaches the ground.

Instead of thinking of the cloudkill as a cohesive ball of fog, I'd think of it as constantly being produced by the centerpoint and evaporating at the edge of the area of effect.

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    \$\begingroup\$ "I'd think of it as constantly being produced by the centerpoint and evaporating at the edge of the area of effect." that is an excellent way of describing it! \$\endgroup\$
    – Tiger Guy
    Aug 31 '21 at 14:52
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    \$\begingroup\$ Shouldn't it be 'the center will move 10 feet straight down' and 'continue to fall at 10 feet per turn' instead of 20, per the movement rate in the spell description? \$\endgroup\$ Aug 31 '21 at 20:12
  • \$\begingroup\$ What if the centerpoint has support but most of everything else doesn't? For example, if I cast it so it rides along a 5ft beam with drops left and right? \$\endgroup\$ Sep 1 '21 at 12:07
  • \$\begingroup\$ @TimSparkles Yes, I wrote 20 by mistake. \$\endgroup\$ Sep 1 '21 at 13:31
  • \$\begingroup\$ @ThanosMaravel In that case my reading would be that it sweeps out a 20-foot area on either side of the beam and does not fall to the floor below, but I could see ruling that the fog doesn't extend beyond the edges of the beam, instead pouring over the sides continuously and evaporating in mid-air 20 feet down. I definitely don't think the intent of the spell is to have the fog affecting two different areas separated by a large vertical distance, or have the center pass through the beam just because the edges of the cloud are unsupported. That would make cloudkill very complicated. \$\endgroup\$ Sep 1 '21 at 13:40
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The spell description explicitly says it will "pour down openings", so, yes, a hole is a suitable defense assuming there is enough volume to accomodate the whole cloud.

In the tower situation, I'd assume the cloud would descend to all lower levels until it finds an airtight barrier. The cloud moves at a 10'/round rate, so it will slowly cross the tower to the bottom, probably affecting at least once everyone in the area.

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    \$\begingroup\$ Essentially, the issue is that 'pouring' could be read as either 'drains away' or 'fills the area down there too'. First feels more intuitive, until you start thinking about origin points and whatnot. Does the origin point move through the floor just because there's a hole at the edge of the spell? What if it's placed parallel to the edge of a clif, so there's solid rock where the point of origin would go even if the edge of the gas falls off? \$\endgroup\$ Aug 31 '21 at 12:12
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    \$\begingroup\$ For this spell I prefer the older edition concept of preserving volume. Assume the cloud is made of some number of 5' cubes and then move them around rather than have it keep its spherical shape. Unfortunately this is not how it has written, but as you notice the spherical shape does not mix well with the cloud draining through openings. \$\endgroup\$
    – Rad80
    Aug 31 '21 at 12:36

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