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My friends and I are currently playing the Princes of the Apocalypse D&D campaign. After a few events, we found ourselves in the Rivergard Keep fighting with a fathomer.

Fathomers are blue humanoids with a shapeshifting ability described as follows:

The fathomer can use its action to polymorph into a Medium serpent composed of water, or back into its true form. Anything the fathomer is wearing or carrying is subsumed into the serpent form during the change, inaccessible until the fathomer returns to its true form. The fathomer reverts to its true form after 4 hours, unless it can expend another use of this trait. If the fathomer is knocked unconscious or dies, it also reverts to its true form.

In our party, we have a level 6 cleric who has access to the spell Create or destroy water described as follows:

Destroy Water. You destroy up to 10 gallons of water in an open container within range. Alternatively, you destroy fog in a 30-foot cube within range.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, you create or destroy 10 additional gallons of water, or the size of the cube increases by 5 feet, for each slot level above 1st.

So, our wise cleric decided to cast the destroy water spell with a second level spell slot on the Fathomer. Not knowing what to do with this, our master decided that the fathomers would lose a lot of HP and turn back to its original form. Still, that left us wondering what to do with this special case.

So, here's my question : How does the spell "Create or destroy water" affect a shapeshifted Fathomer?

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RAW, it seems you quoted the relevant part of the rule already.

Destroy Water. You destroy up to 10 gallons of water in an open container within range. Alternatively, you destroy fog in a 30-foot cube within range.

The serpent form is not water in an open container or fog, and again RAW, should not be subject to damage from the spell. Spells don't generally do more or less than they say.

That said, this also seems entirely in keeping with the idea behind the spell (the spell destroys water and the target is water, a stretch but not by far), and D&D 5th Edition is by design more open to DM interpretation than previous editions. This seems like a case of the DM deciding to reward a player for creative use of their abilities, and s/he resolved that use appropriately.

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  • \$\begingroup\$ Fog is not in a container, and _is water_(in droplet form), and can be destroyed. The transformed fathomer is not in a container, but is water (in denser form) so it makes sense that the fathomer can be destroyed because fog can be. However, having the spell do damage so that it reverts to humanoid form looks like a very smart ruling where no specificity is available. \$\endgroup\$ – KorvinStarmast Oct 8 '15 at 15:17
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    \$\begingroup\$ True, but the spell specifically calls out the fog case as an exception to a general rule, and I think the natural-language use of 'water' refers to that substance in a condensed liquid state. your interpretation may very well work if the DM is willing to rule the serpent form as a dense mist instead of an angry, lethal puddle. \$\endgroup\$ – Eladri Oct 8 '15 at 15:33
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    \$\begingroup\$ FWIW: A puddle need only be a foot deep to be lethal enough to drown someone. 8^O \$\endgroup\$ – KorvinStarmast Oct 8 '15 at 15:35
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    \$\begingroup\$ @KorvinStarmast Fog is “alternately”. The open container requirement is unambiguously attached to water, not fog. (Don't confuse yourself by reasoning that fog is water. If the molecular meaning of water was intended, no mention of fog would be required. Notice neither ice nor steam are mentioned, so the spell affects neither because words mean things.) \$\endgroup\$ – SevenSidedDie Oct 8 '15 at 16:28
  • \$\begingroup\$ @Eladri I like the way you see things. If we were BY THE BOOK, I guess that the spell would have failed, but the good laugh we had after that crazy idea was worth it and the reward was too. Thank you for your insight. \$\endgroup\$ – KeineMaster Oct 8 '15 at 16:39
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As Written...

This is a case where the rules as written say nothing - and leave it to the DM.

Use The Tools In The DMG

The DM, however, has good tools to resolve it. In the DMG, there is a table on page 284 that give the expected damage by spell level....

Create/Destroy water is 1st level, and area effect, so it should use the "Multiple targets" damage... which is 2d6.

So, the Fathomer saves vs the spell's save DC. Con seems appropriate. Damage 2d6, halved on save, +1d per additional level.

I'll reproduce three lines of the table:

Spell    One     Multiple   
Level    Target  Targets
Cantrip  1d1O    1d6
1st      2d10    2d6
2nd      3d10    4d6

Limits

If you feel that too potent (it's roughly the same damage as Magic Missile), drop it to 2d4 or 1d6.

If it feels too weak, the best magic spell at first level is around 1d10+2d6... Ice Knife. The direct damage d10 is behind a to-hit roll, and is lower than the peak, while the area effect is behind a save for half, and is 2d6...

Expected damage average should be above 5.5 (cantrip level) and below 16.5 (2nd level for single target, and 3.5 and 14 for multi-target.

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  • \$\begingroup\$ +1 for citations. Destroy Water has a super specific target restriction, so I'd expect it to better than average 1st-level damage here, but that's a minor complaint. \$\endgroup\$ – the dark wanderer Oct 9 '15 at 17:14
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The Fathomer is 'water in an open container' (i.e. the room), and should be subject to hp loss or death. Your GM's ruling is appropriate, particularly given that 20 gallons of water-loss is significantly less than the Fathomer's probable total volume while in water form. It is not necessary that the container be completely filled, just that it be open and that the affected substance be water, not merely something wet or liquid.

The only change I'd make is that I'd probably allow a Con or Cha save for half damage.

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    \$\begingroup\$ Exactly. It's funny to see how dramatically people read that part of this spell. The water is in an open container as opposed to a sealed container. Everything in the universe is in an open container. \$\endgroup\$ – Preston Oct 9 '15 at 14:09
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The fathomer, in aqueous form, is not "water in an open container." This precludes the fathomer from being targeted, so the spell has no effect.

This, by the way, is the same reading that disallows your cleric from targeting any humanoid as containing ~10 gal. water, and thereby using Create or Destroy Water as a functional Power Word: Kill.

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    \$\begingroup\$ @ nitsua60 This is funny, because we interpreted the open container part as a way of saying "water that you can reach". Technically, the container would have been the room, but I guess this was a bit too far-fetched. \$\endgroup\$ – KeineMaster Oct 8 '15 at 15:13
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    \$\begingroup\$ This prevents targeting water that isn't part of a mixture or solution, so that you can't dry out a humanoid. The creature under consideration is completely made of water, so your reading of this may be overly semantic. The spell having no effect, when fog can be destroyed, does not make sense because fog is water that can be destroyed. \$\endgroup\$ – KorvinStarmast Oct 8 '15 at 15:15
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Destroy Water. You destroy up to 10 gallons of water in an open container within range. Alternatively, you destroy fog in a 30-foot cube within range.

It is my opinion that Rules as Written and Rules as Intended would rule out the possibility of an auto-kill against a Water Elemental. Assuming the DM is using the stat-block from the MM, Water Elementals are Huge according to the stat-block; Maximum volume create/destroy water can effect is 90 gallons. DM's interpretation is required here, but I would say your typical water elemental contains more than 90 gallons of water (If cast using a 9th level spell slot). Substituting the effects for damage like in the case of Inflict Wounds is a good alternative and gives the chance for Rules as Fun to find a place, without breaking the system.

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