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Recently, I was running a game where the party was flying in an urban environment to face off against a wizard. They were all bunched together next to a building about 20' up to avoid attacks from the wizard's constructs but were right next to a wall.

I decided to open with Evard's Black Tentacles using the wall behind them as an anchor point. The party balked saying that the Black Tentacles only went up. This didn't surprise me considering my action would have wrecked the party but I was left wondering later if I got situationally convinced.

3D in D&D has never been my strong suit but the only clue in the spell itself seems to be the word "underfoot". One could argue either way that if you're "standing" on a wall (flying, perfect) the wall is "under" your feet. This is only added to by the confusion that there's no facing in D&D, so flying everywhere could be under your feet=) Thus in my logic, I went with the surface, which a wall seems no worse than a floor.

Is there another reference anywhere that might clear this up?

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I was unable to find any reference beyond the RAW description of the spell, but I think it's reasonable to assume it can be cast on a wall. My reasons are as follows:

RAW Explanations:

  1. Underfoot is hardly a precise description. Given a creature's location, this could be a ceiling or a wall - so this restriction does not stop you.
  2. The fact that the spell can be cast on water demonstrates that the source-surface is not required to be either "flat" or "horizontal", given that water commonly does not have to live up to these requirements (think waves).

Meta Explanation:
3.5 is hardly known for restricting what players can do. Given that Evard's is on the lower end of "optimized caster options", a fair DM would be hard-pressed to limit it in this capacity. Plus, consider it from a caster's point of view. He or she is magically assembling and maintaining these tentacles from the fabric of creation - does it really seem likely that orientation is the core issue?

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The area is a 20' radius spread.

The general rules for magic (in the PHB) state:

The default shape for a burst effect is a sphere.

and

A spread spell spreads out like a burst but can turn corners.

So the effect here is definitely three dimensional.

Now, the spell states:

These waving members seem to spring forth from the earth, floor, or whatever surface is underfoot—including water.

Which doesn't state anything about what direction the tentacles emerge from, only what surface they emerge from.

Let us imagine a homogeneous surface well described by the function z=1/r^2 in cylindrical coordinates. This surface becomes vertical at the pole. Let us imagine the spell is cast by a caster living on the surface at (1,1,π), and that the area of the spell includes (1,0,0), ignoring line of effect issues. The surface, being underfoot, is affected, even the parts of the surface which are vertical, since it is the surface, not the surface's orientation at all points in space-time, which is specified by the spell.

Thus:

The described graph

and after casting:

The described graph with black tentacles sloppily appended

(please disregard the gross errors in axis labelling and choice of coordinates, they are irrelevant here)

The surface, must, however, still be underfoot when the spell is cast. Fortunately, gravity is well-defined in D&D 3.5, and so discerning this can be done easily: surfaces facing away from local gravity are 'underfoot'. Thus the answer to the example question depends where in the multiverse you are:

Plane of Air: Does the caster think the wall is underfoot? He makes a DC 16 Wis check as a free action (retryable each round with a cumulative +6 bonus per consecutive failure), and if he succeeds this works. If he fails, he loses the spell, though.

Material Plane: Down's towards the Earth or other dominant local source of gravity. The walls are not floors, so the spell fails. If the walls are slightly tilted but remain parallel, one of each pair of opposite walls is also a floor and the other one is also a ceiling. Probably a little better than a 50/50 chance this works, since buildings are more likely to be narrow at the the top relative to the bottom than the other way around.

Abyss: The Abyss decides if it wants it to work. You're casting Black Tentacles, but you probably end up with something more like Evard's Spiked Tentacles of Forced Intrusion. Also the Abyss probably likes you slightly more. All in all, a good place to do this.

The Outlands: The wall is a wall, not a floor. The Outlands has an objective up and down and even if a city is built sideways and every inhabitant has spider climb they still know what way 'down' is. Even if you're in space, down is down is down. Down does not change; it is merely your perception of Down that changes. Down is truth. Down is life. Your players are correct and your spell fails.

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  • \$\begingroup\$ With admitted bias: I'm wondering if your answer (material plane) changes at all if there's a windowsill in the wall. Or if there's a brick that's more outset than the other bricks in the wall. Or a flagpole or planter outhanging from the wall. All these could provide surfaces "in the wall" affected by gravity to be targetted but are minuta not often described in D&D (other than by the GM:P)...and maybe then beg the definition of "surface". Sorry if this is a borderline (or not borderline) second Q rather than a clarify request but I'm really interested in the naysayer opinion (aka not mine). \$\endgroup\$ – joedragons Apr 23 '15 at 21:39
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    \$\begingroup\$ @joedragons Yes, those surfaces are definitely targetable. However, it is unlikely that the planter box would be part of the same surface as the wall. The same for the brick. What you need is part of the same surface to go floor-like. The windowsill's your best bet, but you'd want it not to have an edge. \$\endgroup\$ – the dark wanderer Apr 23 '15 at 22:01

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