Can players choose specific points in space, down to the inch, to cast a spell so as to avoid hitting a prone character?

Question

Say there is a character that is prone, such as if they were unconscious, and they are surrounded on all 8 square grids (assuming that a grid is being used) by other creatures. Can a player cast a spell that has a sphere effect such as fireball or shatter such that only the 8 creatures surrounding the one that is prone be hit?

Would this potentially have any adverse effects with potentially breaking or having any unintended consequences for any other spells/effects down the line if this were allowed?

Obviously when it comes to casting some spells, the caster has the option to "choose a point in space", but when playing with the understanding of a grid system that works in chunks of a given dimension does is it feasible to have spells cast in such a way so that a body lying prone won't be affected by a spell cast just overhead?

Answer 1

I'd have to say no.

For the 1st question:

Can a player cast a spell that has a sphere effect such as fireball or shatter such that only the 8 creatures surrounding the one that is prone be hit?

Since we're assuming a grid is being used, from the DMG page 251 we have

If an area of effect is circular and covers at least half a square, it affects that square.

Under that rule, to avoid the prone creature (assuming size medium), we'd have to have less than half of the 5' cube they control included in the sphere. (The rules don't actually specify the height of grid squares, but for the sake of argument I'm going to assume 5' cubes in 3 dimensions, as discussed in this question.)

Mathematically, if you center a 20' sphere directly above the center of the 5' square controlled by the prone creature, and you set the height so that less than half of that creature's 5' cube is covered by the spell, then the adjacent cubes will have even less of their volume covered (the sphere curves upward as you go out) and would not be affected either.

The 2nd question is moot for me, since I wouldn't allow it.

For the 3rd question,

...is it feasible to have spells cast in such a way so that a body lying prone won't be affected by a spell cast just overhead?

As discussed above, to do so you need to position the spell so that less than half the area the prone creature controls, nominally a 5' cube, is included in the spell.

As an aside, if I'm casting fireball and trying to hit as many of the 8 creatures surrounding a single friendly creature as I can, then prone doesn't really help me. The best you can do is have your spherical area contain just shy of half of their space, which would encompass 3 of the opponents, either 3 on one side of them, or 3 at and next to a corner of their space, depending on where you center the fireball relative to them.

And lastly, back to question 2, should you allow any caster of a fireball or other spherical AoE spell to spare a creature at the center who drops to the ground? Something to consider is that you're providing a feature that is similar to and in some ways more powerful than certain class-specific features.

For example, the Sorcerer Metamagic Careful Spell (PHB p. 102) allows the Sorcerer to spend a sorcery point to allow some creatures to automatically succeed on their save (they would still take half damage from a fireball).

Also, the Wizard School of Evocation ability Sculpt Spells (PHB p. 117) allows the wizard to choose certain creatures to automatically succeed on their saving throws against the spell, and they take no damage if they would normally take half damage on a successful save.

Allowing anyone casting spells to achieve an effect as good or better than class abilities can somewhat undermine some of the uniqueness of your players class choices.

Answer 2

Seems like it's up to DM discretion, according to Jeremy Crawford's tweet here at least.

The way I'd handle this personally would be to allow it but maybe give the 8 creatures advantage on their dex saves or something, because targeting an aoe damage spell with the specific intent of hitting targets that are only partially in range and only barely at that seems like something that should be discouraged at least somewhat, because it seems like something that ought not to work as well as using a fireball "normally."

Answer 3

Yes, if the DM allows it.

The Sage Advice Compendium discusses the positioning of AoE effects:

Using 5-foot squares, does cloud of daggers affect a single square?

Cloud of daggers (5 ft. cube) can affect more than one square on a grid, unless the DM says effects snap to the grid. There are many ways to position that cube.

Under Areas of Effect on page 251 of the Dungeon Master's Guide, it reads:

If an area of effect is circular and covers at least half a square, it affects that square.

However, this doesn't really address the fact that creatures have a height and that this height will often extend above a 5' vertical increment. Calculating three-dimensional areas of effect can be very difficult.

In such instances, a DM would have to adjudicate how to handle, for example, the fact that a monster's head and shoulders extend above a 5 x 5 x 5 cube into the next one above it that is affected by an area effect.

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At my table

At my table, I allow players to attempt to place spells in very specific locations but require that they make an Intelligence check to place the spell accurately. Depending on how they roll, the spell either ends up exactly where they want it or a little off target in the middle of a space of (usually) their choice.

I make it a flat Intelligence check without proficiency because I figure this roughly captures the idea of their character trying to calculate correct range and bearing to the target point. Plus, it incentivizes players of Wisdom/Charisma spellcasting characters to not just dump Intelligence.

The DC is 12 + spell level because I find this tends to scale the difficulty about right. The higher level the spell and greater its effect, the harder it is to control that amount of magic accurately.

Any creature whose height or width would put them half way or more into a space is affected. Otherwise, the effect's potency trails off by its fringes and has no mechanical effect. I often give the players some sort of narrative result if they don't actually get the mechanical result they truly want.

For example, if they try and take out a bunch of zombies around a halfling ally by placing the fireball so that it just snags the heads of the taller zombies but miss the placement, I'll let some of the zombies' hair be burned off.

Answer 4

While this is going to strongly depend on DM discretion, one thing some of the other answers are ignoring are the height of the surrounding creatures. If a character is prone and surrounded by creatures who are at least 6 feet tall, I will absolutely allow a fireball or other spell that allows a "point in space" choice to hit the creatures who are five feet closer to the blast than the prone character is. I may require some kind of check (if the PC doesn't have Engineering or Architecture or some other proficiency that would cover eyeballing distances accurately, might just be a straight Intelligence check), or I might not, depending on game and circumstance, but I would definitely allow them to try.

Sorry this is a rules-light answer, but grids are already an optional rule in 5e and never took height into account when written. But absolutely should.