Traits

Antimagic Shell. When a spell is cast targeting the Flail Snail, the caster makes a DC 15 spellcasting ability check. Failure: The spell fails and has no effect (the spell slot is still expended).

This doesn't stop area effects (such as Fireball) — it only affects spells that directly target the Flail Snail.

Shell Shield. When the Flail Snail withdraws into its shell, its AC becomes 20, but its Speed becomes 0 and it can't make attacks.

Slippery Trail. Squares the Flail Snail passes through become Difficult Terrain (for 1 minute).

Actions

Multiattack. It makes five Flail Tentacle attacks — they don't all have to target different creatures, but no more than two can target the same creature.

Flail Tentacle. Melee Attack Roll: +7, reach 10 ft. Hit: 9 (1d10 + 4) Bludgeoning damage.

Scintillating Shell (Recharge 5–6). The Flail Snail's shell flashes dazzling colors. Every creature that can see the shell and is within 30 feet must make a DC 15 Constitution save. Failure: The target is Blinded for 1 minute. The target can repeat the save at the end of each of its turns.

Bonus Actions

Withdraw. It withdraws into or emerges from its shell (toggling Shell Shield).

How to Use

The Flail Snail is a creature that punishes casters but rewards martials — the reverse of most 5e encounters, and it adds variety to the table.

Antimagic Shell is key: if the Wizard wants to cast Fireball — that area effect goes through. But if they cast something that directly targets, like Hold Monster or Disintegrate, and fails the DC 15 check, the slot burns and nothing happens. This forces the caster player to think.

Potential of five attacks × 9 damage = 45/round. But with Speed 15 ft. and reach 10 ft., the party can retreat and kite. Fair.

Scintillating Shell is the real danger — everyone within 30 feet can be blinded. A blinded party fighting something that deals 45 damage/round is the turning point of the encounter.

Balance Note

Don't overuse Antimagic Shell. "The slot is spent but nothing happens" is a harsh penalty. If your table's caster player genuinely dislikes this, use this softening: on a failed check, the slot isn't spent — the spell simply has no effect. The sense of threat remains, but there's no resource loss.

For a 4-person Lv6 party, a single Flail Snail = 1,800 XP, budget (Medium) 1,000×4 = 4,000. Low–Medium. You can use two, or pair it with a few weaker creatures.