[RPG] When casting the Control Water spell underwater, does the Part Water option create a cube of air

3d-spacearea-of-effectdnd-5espellsunderwater

The control water spell has the option to separate water and form a trench on a cubic area of up to 100 feet per side:

Until the spell ends, you control any freestanding water inside an area you choose that is a cube up to 100 feet on a side. You can choose from any of the following effects when you cast this spell. As an action on your turn, you can repeat the same effect or choose a different one.

[…]

Part Water. You cause water in the area to move apart and create a trench. The trench extends across the spell's area, and the separated water forms a wall to either side. The trench remains until the spell ends or you choose a different effect. The water then slowly fills in the trench over the course of the next round until the normal water level is restored.

This is easy to imagine when cast at the water's surface. There is air above the water, and so parting the water forms a cube-shaped trench with breathable air.

What happens if the targeted area is all underwater, not near the surface? Does parting the water create a cube of air?

Best Answer

Who's to say?

The DM, that's who. Naturally, when the trench created opens up to the surface, the area is filled with the air from the above the water. But when the trench created is entirely underwater, the description gives no guidance as to what happens. Does it created a vacuum? Does the water column above the void drop down, filling the trench? Does it create a pocket of breathable air? The spell description does not give anything with which to begin answering those questions, so the DM needs to make a ruling.

If you have control water, and anticipate going on an underwater adventure, talk to the DM ahead of time. Have a conversation and work out together what the spell is going to do, so that your expectations are not squashed when the DM rules differently than what you thought. If you know a particular spell interaction has potential for confusion, work with your DM ahead of time so they don't have to make the ruling on the fly. And if it comes up, don't be afraid to have a review conversation with the DM. Did this ruling work well? Should we use this ruling if it comes up again?

This can get surprisingly complicated.

So the first point is, you don't need to use any grid at all. In that case, rotating the cube is natural and you are right with the \$L\sqrt 3\$ diagonal case.

If you do use the grid however, it gets muddy. Dnd measures distances not in normal way, but just as a maximum of coordinates (this is a weird way of saying that diagonal movement costs the same as non-diagonal).

So either you sort of rotate the grid temporarily (in your head) to figure out the area of effect, in which case it works the same as the fist case, even though now you distance from the target is only \$L\$.

Or the grid stays and then you start studying the theory of metric spaces to figure out how does a cube in this situation even look like. The easiest to imagine would be the 45° rotation (2D) case, in which (C- caster, T-Target):

   X
  XXX  
CXXXXT
  XXX
   X

Or to be more srict with the rules about origin and faces (the caster is by the south-west face facing towards north-east):

   X
  XXX  
 XXXXT
C XXX
   X

Notice how the diagonal is 5 squares (or 25 feet) long, while the side is 3. But this feels a bit cheatsy since the concept of cube here is not very well defined.

Finally, note that DMG p.249 has rules/guidelines on how many targets does each shape "usually" hit for theater-of-mind and similar approaches. For Cube it states size/5 (round up), so 15 ft cube gives you around 3 affected targets (in situations where exact positioning is not a top priority).

[RPG] What are the limits of the Flood option of the Control Water spell

A barrel of water is an invalid target for Control Water

Control Water states that you must target freestanding water:

Until the spell ends, you control any freestanding water inside an area you choose that is a cube up to 100 feet on a side.

Freestanding water is natural water in lakes, rivers, swamps. Read this answer for more information. While a manmade pond or dam might create questionable situations for your DM to resolve, a barrel of water is clearly not a valid target.

As such when Control Water states:

If the area includes a shore, the flooding water spills over onto dry land.

It is referring to the land surrounding the lake, river, swamp, etc. That is why the wording seems so confusing when you target a barrel, you are using it in a way that is not allowed.

If you target a valid body of water the spell is explicit about what happens, the water level rises and the flooding water spills over onto dry land.

As for your other query:

Does this create new water, or simply levitate the water already present?

You can use the Polymorph spell to transmute a human into an elephant. Even though an elephant has more mass than a human, the spell magically performs the task. There is no real explanation as to how this works, it's magic.

It's the same with Control Water. Nothing is ever created. The water is transmuted from the existing water. The water does not levitate, only the water level rises.

Best Answer

Who's to say?

Related Solutions

[RPG] How does orienting a cube-shaped spell work in three-dimensional space

This can get surprisingly complicated.

[RPG] What are the limits of the Flood option of the Control Water spell

A barrel of water is an invalid target for Control Water

As for your other query:

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