[RPG] Help with 3D combat


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.

Best Answer

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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