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

area-of-effectdnd-5espells

Can the caster orient the cube any way they like? Can the point of origin effectively be a corner, with the opposite corner pointing any way the caster chooses?

This question was tagged as a duplicate; however, the linked question and its answers presuppose that the cube has a face parallel to the ground. In that question, the "cube" is effectively a square on the ground, plus L feet of airspace above the square (where L is the length of a side of the square).

This question challenges that assumption. I read the rules, and I took them to mean that a cube-shaped spell was an invitation for creative use of three-dimensional geometry.

I get that some groups will ignore or fail to notice this invitation (not every group will enjoy geeking out over geometry). My question, though, is whether this creativity is actually prohibited by the rules. Is there a rules-based argument to prevent, for example,a cube-shaped spell from hitting a target R + L*sqrt(3) feet away, where R is the spell's listed range and L is the listed length of the cube's side?

The description of a cubic area of effect is given as follows (PHB. 204, or here in the basic rules)

CUBE

You select a cube's point of origin, which lies anywhere on a
face of the cubic effect. The cube's size is expressed as the length
of each side.

A cube's point of origin is not included in the cube's area of effect,
unless you decide otherwise.

Can the cube's point of origin be at the corner or edge of the cube– or at least arbitrarily close to the corner or edge of the cube? Even if we parse "face" as "not including edges or corners," surely the caster can declare that the point of origin is a picometer from the edge or corner?

Can the caster orient the cube at funny angles? I don't see any rule saying the cube can or can't have whatever orientation the caster likes.

Take thunderwave (PHB p. 282) as an example:

A wave of thunderous force sweeps out from you. Each creature in a
15-foot cube originating from you must make a Constitution saving
throw. On a failed save, [things happen].

How close does a caster have to get to a single target to hit them with thunderwave? The way I figure it, a caster can set one corner of the cube as the point of origin, then orient the opposite corner in the direction of the intended target.

If I did the math right, the distance across the diagonal of a cube is L*sqrt(3), where L is the length of a side of the cube. Hence, thunderwave could hit a creature as far as 25.98 ft from the caster (because 15ft * sqrt(3) is approximately 25.98 ft).

This would cause part of the spell's Area of Effect (actually, it's a volume) to be blocked by the ground.

Best Answer

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).

Unfortunately, no (with one possible exception).

Although this isn't the place to say perhaps, I think I have a slight contention with the answers to the related questions on whether the caster including themselves in the area of effect, is completely inside the area in the way portrayed. I see this more as just the point of origin affecting the caster (as it says) but not that the entire area of effect has shifted.

Typically this would be so the caster can affect himself with a beneficial effect, which is likely the intention. In other words, the area of effect projects out in front (see the diagram below), but the caster, right on the point of origin, can be affected or not.

 xxx
cxxx
 xxx

Nevertheless.

Even accepting the ruling given on the previous question, your scenario would only possibly work against larger creatures.

Either you are in the area and hence affected by the spell. Or you are not, which would mean the only way you could be in the center square of a cubic area of effect, yet not affected, would be if the cube was above you (or below you if you were somehow flying?). In this case a DM may rule it would only affect creatures that are taller than you, and tall enough to be affected by the blast that has just gone off above your head.

Nahyn Oklauq points out that it would be possible to lie down and cast Thunderwave from the ground, hence affecting same-sized creatures. This is playing a bit fast and loose with the rules (which are really geared towards a 2D map rather than these kind of 3D shenanigans), so would probably be DM fiat as to whether he would allow it.

Best Answer

This can get surprisingly complicated.

Related Solutions

[RPG] How many squares on a 5-foot grid does a 5-foot radius spell affect

[RPG] cast Thunderwave and be at the center of its bottom face, but not be affected by it

Unfortunately, no (with one possible exception).

Nevertheless.

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