I recently asked this question, in which one of the comments mentioned there might be problems with 3D combat. My campaign will be nearly entirely underwater, in the deep sea where 3 dimensional movement is normal.

I'm looking for any potential problems that running 3D combat encounters will bring, and how to deal with them.
The game hasn't yet started, and I've never run 3D combat encounters before. My main concern is that all the flat AoE spells will become useless, since any target can move off the 2D plane that the spell is on and get out of the effect.

I'm not looking for RAW answers only. Any house-rules or homebrew spells and abilities would all be appreciated here.

  • \$\begingroup\$ I understand how underwater combat functions mechanically, but I think that it'll be much harder since NPCs will be much more spread out. And I'm not entirely sure what to ask. I'll try and rephrase. \$\endgroup\$ – Timi Dec 20 '19 at 21:15
  • \$\begingroup\$ What is a "flat AoE spell"? \$\endgroup\$ – Szega Dec 20 '19 at 22:43
  • \$\begingroup\$ Something like Entangle. \$\endgroup\$ – Timi Dec 20 '19 at 22:50

Matt Colville has a good video on this topic: Range and Altitude in Three Dimensions, Running the Game #55

It notably advises:

  • In 5e, measuring range in three dimensions is easy because of how 5e handles diagonals. If something is 5 squares away in one dimension and 3 squares away in another, it's always simply 5 squares away.
  • Use d12s next to a miniature to measure how many squares vertically up someone is. You're rarely more than 12 squares vertically up, and d12s don't get much use.
  • There are specialist miniature stands available to represent height. One idea is a vertical rod, presumably on a base and marked with distance increments, to which the mini is clipped using a small crocodile clip.

You'll also need to consider the underwater combat rules, Dungeon Master's Guide p.116-119. Notably:

  • Characters without a swimming speed (e.g. normal humans) can suffer exhaustion when swimming over long periods.
  • Visibility is reduced underwater. Even in clear, brightly-lit water, one can often only see to a distance of 60 feet. This limits the range at which combat can happen, so very spread-out battles are unlikely. It also creates opportunities for stealth, but potentially requires more work regarding fog of war, i.e. determining who can see who.

And in the Player's Handbook, p.198:

  • Creatures without swimming speeds have disadvantage on melee attacks made underwater, except with stabby weapons (dagger, javelin, shortsword, spear, trident).
  • Ranged weapons automatically miss beyond normal range; there is no long range.
  • Ranged weapons have disadvantage, except for crossbows, nets, and javelin-like weapons such as a spear, trident or dart.
  • Creatures immersed in water have resistance to fire, reducing the effectiveness of fire spells.

I would imagine underwater civilizations would develop more effective magic and tools, such as waterproof spellbooks (or an alternative to spellbooks), and variants of major spells (such as a cold or acid version of fireball, non-flat versions of area spells, and spells which are equally effective when you're not at ground level).

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I've had a couple of different types of 3D combat setups, and based on my experiences I have a few suggestions. The first, and by far the most important, is to remember that:

D&D was not built for 3D combat any more than it was built to exactly simulate reality.

This doesn't mean that you can't or shouldn't try out 3D combat, just that you should treat it like any other abstraction in the game. Hit points do a terrible job of precisely simulating a creature's ability to continue living or be in good shape, but a great job of giving fun, reliable structure to combat encounters. A Charisma check to persuade someone to do something glosses over a lot of details that would apply in reality.

That means that some of the most obvious ideas might be better abandoned. For example, I tried keeping track of how many squares above the ground characters were.

  • This is difficult without some sort of 3D grid (I'm not aware of any such thing, either)
  • It is really hard to remember which areas, exactly, are under the effects of AoE spells (like Healing Spirit). Most AoE spells have areas of effect that are described in three dimensions already, but height is generally not a complicated concern in 2D combats
  • It makes things like resolving distances much harder than the base rules (instead of just counting squares, you now have to do some trigonometry all the time)
  • It also does a lot of damage to important game concepts like cover (creatures have more options for getting around it) and you have to adjudicate situations on-the-fly such as what angles of attack make cover less effective
  • It radically changes the balance among characters with different abilities, heavily favoring ranged attackers (there are more "squares" creatures can be in, while melee range doesn't increase at all)

There are more issues, but these examples give the flavor well enough. The solution, if you really want to include 3D maneuvering, is about abstractions.

Thinking about elements which simulate but do not duplicate the "real world" effects of operating in three dimensions. The elements you simulate should give some 3D "feel", and more importantly, provide new tactical considerations.

I don't know your specific goals for combat and difficulty in this campaign, so I'll offer some broader suggestions:

Movement and Maneuvering

  • Decide on a pre-defined number of layers of depth for a combat encounter. I've based this on attack ranges (my player's flying warlock deals with bowshot normal range/bowshot disadvantage range when fighting with characters with bows, and Eldritch Blast range for his own attacks, for example). There are no gradations in between. This makes 3D maneuvering less rich, but radically simplifies and speeds up the majority of adjudications you have to make. The layers can also be relative to where characters are-- there is no fixed bottom layer, and if all characters move up one layer exactly it's as if none of them moved at all, vertically
  • Players should be expected to keep track of their vertical position. It's an odd match to other mechanics, and leaving it all to the DM is a bit unfair (it's a lot of extra work), and leads to disputes more often than you might expect ("I thought I was here, not there!"). Players that forget should never gain an advantage from having done so. Props may help with this, but I've not used any
  • Think in advance about how you want to handle vertical movement speed. Moving by layers doesn't work very well with some frames of reference (missile attack range in particular is hard) because it allows characters to casually move out of range of attacks in ways that would be totally impossible in 2D. Options include using Actions to change layers (like a vertical dash), using arbitrary amounts of Movement (like getting up from being prone), tracking vertical distance square-by-square and requiring multiple turns' worth of movement to change layers, and others
  • If you do want to count individual squares for measuring vertical distances, make a cheat sheet for yourself. Something along the lines of "X squares horizontal distance and Y squares of vertical distance equals Z Euclidean distance". Basic trig isn't that hard, but checking a table is even easier (as well as faster and more reliably correct). You'll probably find that you only actually encounter a few combinations of effective distances, and so you may not even need to use the cheat sheet very often

Making it Interesting

  • The y-axis plane should be similarly interesting relative to the x-axis plane. Most often I find that means cover between vertical layers needs to exist. Without this combat will essentially rotate 90 degrees, operating on a 2D plane which is arbitrarily different from and harder to work with than just using a 2D area
  • Where appropriate, 3D tactics need to be used by NPCs and enemies. Simply giving the PCs three dimensions to work with and then playing enemies as though they only have two makes the game wildly easier for the players. If an enemy can be engaged by ~26 PCs at once, the PCs should get swarmed that way as well (at least sometimes)
  • There should be real obstacles to movement in some encounters. My player's flying warlock sometimes finds himself indoors, where flying isn't feasible or he just can't get very far off the ground. 3D should add more tactical situations, not just make combat harder to run

Recognize Likely Mechanical Problems

  • Decide with your players how you will rule on each spell they might take at character creation so that there are no surprise nerfings. Many spells are trivial to port to 3D. Entangle can be a mass of seaweed or a cloying cloud of algae and provide exactly the same properties as on a 2D plane, particularly if you aren't counting each vertical square of space (which you should really consider avoiding)
  • Forget about CR calculations from the book. They don't take any 3D anything into account, and so you will want to game out combats yourself and also have backup options for if fights become too difficult or too easy
  • Remember that D&D 5e was not built to do this, and so some changes may be unavoidable. The combination of factors you settle on for your 3D combat system may break some specific spells, or require an arbitrary, consistent judgement call on exactly how it works. But do not expect the basic rules and mechanics to work as well here as they would with normal combat. Such is the price of homebrew. If this means that certain spells, builds, or even classes aren't really viable in this campaign, you'll have to decide which you want more: those things, or 3D combat

And, the most important thing to do with such a dramatic homebrew rule change as this:

Talk to your players about it. You're imposing some very sweeping changes with a largely homebrewed combat system, and that system is likely to be awkwardly balanced and may or may not match up with gameplay that your players enjoy.

There is no problem in trying something out for a session or two and then finding out that it doesn't work well and your players aren't having fun with it. There is a lot wrong with imposing a system on the game that no one likes, and keeping on with it because you never find that out.

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For one thing, there are no spells that effect only a flat area. Spell targeting types for AoE spells are cone, cube, cylinder, line and sphere. The only other spell targets are self and choosing individual targets.

You best bet would be to use the rules for miniatures and a grid. Use stands to place creatures at whatever elevation they are positioned at. For cubes, spheres and cylinders you can work out which targets will be affected based on their position. For lines you'd need to use some kind of straight object or string to work out what will be in the path. For cones I can't give any good suggestions.

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    \$\begingroup\$ Actually there are spells that target flat planes such as "the ground" for example grasping Vine \$\endgroup\$ – Medix2 Dec 20 '19 at 21:19
  • \$\begingroup\$ Yes. I'm mainly referring to spells such as Entangle. There are a few, and while they're 5ft high or so, they're still flat for the purposes of 3D space, where someone in the centre can get out within 5 feet of movement, as opposed to 20. \$\endgroup\$ – Timi Dec 20 '19 at 21:21
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    \$\begingroup\$ So, cylinders gets more potential to affect a lot of creatures, right? With no height limit, doesn't thee deserve a bit more thought? And where are the rules for 3d grid? Am I missing something? \$\endgroup\$ – Mołot Dec 20 '19 at 23:30
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    \$\begingroup\$ @Mołot The rules for 3D combat are the same as for "2D" combat, they start on page 249 of the DMG. You just need to be more aware of height when it comes to distance and AoE attacks. I believe most, if not all, cylinder type AoEs have a maximum height. \$\endgroup\$ – Allan Mills Dec 21 '19 at 22:11

By itself 3D does not really matter

Here is the thing: your problem will be to make combat any different, if you want a thematic campaign. Introducing the 3rd dimension will not significanty change how D&D combat plays out.

Why? Because the combat environment is nothing but the information of where characters are and where terrain features are. What do players ask the GM about? Can I reach that cover with only my move? Can I get those monsters all in my AoE? It is only about the pairwise distance of things. And you can already be 10 feet from one enemy, 30 from another and 50 from a piece of cover on a 2D map.

What the 3rd dimension will do is that now not only 3, but 4 things can be equidistant from each other, which is pretty much inconsequential for us. It is possible that things that provide substantial cover have to be bigger to do the same job, but most area control AoE-s are already fairly large, and in the case of terrain features, you can literally make them as large as needed. Another effect will be that now more creatures can surround a single creature. But on a 2D map you can already be surrounded by 8 creatures (medium vs. medium on a square grid), and in most combats there will not even be 8+ combatants on one side. Even in "mass battles" with two dozen monsters, I have never once saw a situation where 8+ enemies wanted to surround a single character.

Going underwater

So we can see that there is no significant geometrical effect. Besides the rules of underwater combat outlined in the books (which will likely be mitigated or completely avoided by any party worth their salt or any monster actually living there), there are really only two things that need attention.

Most things sink.

Dropped your sword? Sinks. Was swimming and speed dropped to 0? Sinking. Threw a dagger? Will sink. Depending on how likely it is that you will have the chance to go after things that sink, the attention given to these situations will change.

The most common reason people drop their weapons in D&D 5e is to switch to another and still be able to attack with it on the same turn. In a usual environment you can expect to be able to retrieve it, but that will change here. This will lead to people rarely switching weapons in a combat situation, as it becomes too costly in either actions or money. The switch is usually from a melee weapon to a ranged one or vice versa. Thus characters will be more dedicated to one role or the other.

You are also usually assumed to be able to retrieve thrown weapons if you didn't have to leave the combat site (eg. to retreat or catch a fleeing enemy). Daggers, handaxes and throwing stars will sink after one use, while a javelin might float due to its wooden shaft. Either way, the retrieval of the items will be much more problematic, which will discourage their use.

The above two can be avoided (and they automatically are by most spellcasters), but in combat characters will go unconscious. If you are swimming by your own power, you will sink when you cannot move. So a downed teammate is not only rolling death saves, but also getting farther and farther away from help. This can make it a truly pressing issue, which is not always how it feels in land combat ("I still have time to kill this orc, I can save him next turn all the same."). The lethality of combat might rise a bit.

These are not issues that necessarily need to be "solved", though. Leaving them as is will not break anything, and might even lend a bit of unique feel to the environment.

Open water

On land it is easy to introduce terrain features to combat. Most places have trees or shrubs, and even in a sand desert there are dunes that can provide cover, for example. However, the deep seas are fairly empty, as not many things float at just a given depth apart from fish. A fight on a featureless plane (now in 3D!!!) is alright once in a while, but you will need to come up with a lot of ideas if you plan to run a whole campaign underwater. You could run a combat at the bottom, where there could be bentic flora, but then you are practically running a land fight, so if you want a heavy theme, I would not recommend relying on it too much. I will list two ideas I could come up with quickly, but this is far from enough for a campaign. This is an issue you have to be prepared to tackle.

  • Beside the huge coral reef. They are exotic and cool looking and can really form mountains even in our non-fantasy world. You can take cover inside it or on the other side of it.
  • Hunt for the Red October. There is a new gnomish contraption (a submarine) and the party is tasked with getting it. The submarine can be fairly large to serve as terrain and can float. It can also move, which can have interesting consequences.

Circumstantial abilities

You mention that there are spells that rely on there being ground or air around. Well, there are also spells that only work when there is plenty water around, like control water. Whatever the place, there will be circumstantial abilities that do not work to their full extent or at all.

As your players are aware of the theme of the campaign, they can make informed choices about which abilites they want to take. I would recommend against changing abilities or spells for ease of use. One, this might place the focus on rarely used choices, making the experience different. Two, if you really want to, you can introduce these as rewards ("As thanks for saving their village, the sahuagin shaman offers to teach you the spell iceball.").

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I ran a D&D 5e campaign with some extended sections underwater.

One of my Homebrew rules about spells were that spells with fire and acid damage only caused 1/2 damage underwater (before any resistance/vulnerability was applied), where as lightning and cold spells caused x2 damage (again before any resistance/vulnerability was applied).

There were some mini quests where some players got magical items to help them either with swim speed, water breathing, water walking (surface).

For ranged weapon attacks I reduced their distance by half.

When working out the mechanics of battles and movement I used miniature d10 dice next to each player's tokens to show the depth. For example, if a player with a movement of 30 feet wished to swim down from the surface, they would put the dice on 3. If the went up 10 feet in their next turn, it would be 2. I never had to used a d20 but I guess in battles where they might have been more spread out I could have used this. To work out diagonal travel I simplified it (no pythagoras, lol): 30 feet swim speed = 30 feet across, or 20 across and 10 down, or 10 across and 20 down, or 30 feet down.

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