Does the Flail Snail's "burst of destructive force" do no damage if it's triggered by a cantrip such as Eldritch Blast?

Question

My party encountered a Flail Snail in our adventures, and the Warlock in the party had no real combat ability versus said snail other than the Eldritch Blast cantrip. The Warlock missed their attack, and as such the DM rolled for the Flail Snail's random effect. The way I am understanding this, only on a roll of 1 or 2 would the spell do anything other than miss.

The relevant passage, quoted from the flail snail's statblock (emphasis mine):

Antimagic Shell. The snail has advantage on saving throws against spells, and any creature making a spell attack against the snail has disadvantage on the attack roll. If the snail succeeds on its saving throw against a spell or a spell attack misses it, an additional effect might occur, as determined by rolling a d6:

1–2. If the spell affects an area or has multiple targets, it fails and has no effect. If the spell targets only the snail, it has no effect on the snail and is reflected back at the caster, using the spell slot level, spell save DC, attack bonus, and spellcasting ability of the caster.

3–4. No additional effect.

5–6. The snail’s shell converts some of the spell’s energy into a burst of destructive force. Each creature within 30 feet of the snail must make a DC 15 Constitution saving throw, taking 1d6 force damage per level of the spell on a failed save, or half as much damage on a successful one.

If I am understanding this correctly, that would mean an attack cantrip would only have a 33% chance of a negative effect on a miss, and on a roll of 5 or 6 do zero damage regardless since cantrips are treated as 0-level spells.

Is this interpretation correct?

Answer 1

Cantrips are treated as "level 0" spells

From the description of cantrips in the basic rules:

A cantrip is a spell that can be cast at will, without using a spell slot and without being prepared in advance. Repeated practice has fixed the spell in the caster's mind and infused the caster with the magic needed to produce the effect over and over. A cantrip's spell level is 0.

So, for the purpose of the flail snail: if someone uses a spell attack cantrip on the flail snail and it misses, or someone uses a saving-throw cantrip on the flail snail and it makes the save, then it (or, well, the DM) rolls a d6. However, if it rolls a 5 or a 6, it actually doesn't do anything as written, because cantrips are level-0 spells - 1d6 times 0 equals 0.

Your interpretation is correct, for the rule as written. No minimum damage is specified.

(This may not be the intent, but you'd have to ask Jeremy Crawford - then maybe it'd be fixed in errata to do a minimum of 1d6 damage for cantrips.)

Answer 2

On a 5–6, there is no force damage.

Cantrips have an effective spell level of 0, according to the rules for spellcasting:

A cantrip's spell level is 0.

So if you roll a 5–6 for the flail snail's effect which can deal 1d6 force damage per level of the spell, that calculation is equivalent to 1d6 times 0, which is 0 damage, regardless of the result of the save.

Since 5–6 would effectively do nothing and 3–4 explicitly does nothing, you are correct that only 1–2 could have an effect (which is 1 of 3 cases, or 33% of the cases). However, there is only a practical effect if the reflected attack actually hits the original caster, which means we would expect the probability of a negative effect at the table to be much less than 33%.

Answer 3

The damage is zero, but the effect might not be

You have the correct calculation as cantrips are 0-level spells per the PHB:

A cantrip is a spell that can be cast at will, without using a spell slot and without being prepared in advance. Repeated practice has fixed the spell in the caster's mind and infused the caster with the magic needed to produce the effect over and over. A cantrip's spell level is 0.

However, nearby concentrating spellcasters may need to save for concentration if subjected to the effect. The need for this may vary depending on the DM's interpretation of 'taking damage' as it relates to concentration saves, but is further discussed here.

The basis for a concentration save being needed stems from the following part of the Concentration rules:

The GM might also decide that certain environmental phenomena, such as a wave crashing over you while you’re on a storm-tossed ship, require you to succeed on a DC 10 Constitution saving throw to maintain concentration on a spell.

Some DMs may consider the snail's 'burst of destructive force' to be of a nature that disrupts your concentration. It didn't hurt you (i.e. caused damage), but neither did a wave crashing over a ship; regardless, concentration is a taxing activity and might've been broken by being within the radius.

Answer 4

Cantrips are usually treated as 1st level

In most cases where the numerical value of a spells level is considered, cantrips are treated as 1st level (value = 1).

For example, Sorcerer Twinned Spell

You can spend a number of sorcery points equal to the spell's level ... (1 sorcery point if the spell is a cantrip).

There is no specific guidance for the Flail Snail

The poor flail snail doesn't specifically say what to do with cantrips so you could either treat them as level 0 or level 1, both are valid interpretations and until we get errata or sage advice there isn't a definite ruling.