[RPG] How does the spell “Create or destroy water” affect a shapeshifted Fathomer

dnd-5emonstersshapeshifterspells

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?

Best Answer

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.