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 fire damage per level of the spell on a failed save, or half as much damage on a successful one.

Lava Tentacles
The volcanized snail has five lava tentacles. Whenever the snail takes 10 damage or more on a single turn, one of its tentacles dies. If even one tentacle remains, the snail regrows all dead ones within 1d4 days. If all its tentacles die, the snail retracts into its shell, gaining total cover, and it begins wailing, a sound that can be heard for 600 feet, stopping only when it dies 5d6 minutes later. Healing magic that restores limbs, such as the regenerate spell, can halt this dying process.

Magma Trail
The snail leaves a trail of magma behind it as it moves. Magma will ignite flammable surfaces and uncarried objects within 5ft of it. Creatures moving through, or starting their turns in, the magma trail take 1d6 fire damage. Magma lasts for 1 hour then cools to form rock.

Multiattack
The volcanized snail makes as many lava tentacle attacks as it has lava tentacles, all against the same target.

Lava Tentacle
Melee Weapon Attack:  +5 to hit, reach 10 ft., one target. Hit: 6 (1d6 + 3) fire damage.

Thermal Vent (Recharges after a Short or Long Rest)
The snail's shell emits intense, light and heat until the end of the snail's next turn. During this time, the shell sheds bright light in a 30-foot radius and dim light for an additional 30 feet, and creatures that can see the snail have disadvantage on attack rolls against it. In addition, any creature within the bright light and able to see the snail when this power is activated must succeed on a DC 15 Constitution saving throw or take 1d6 fire damage until the light ends.

Shell Defense
The flail snail withdraws into its shell, gaining a +4 bonus to AC until it emerges. It can emerge from its shell as a bonus action on its turn.

Reaction Name. Enter the description for your action.