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?