As an action or a reaction, you can withdraw into your shell. Until you emerge, you gain a +3 bonus to AC, and you have advantage on Strength and Constitution saving throws. While in your shell, you are prone, your speed is 0 and can't increase, you have disadvantage on Dexterity saving throws, you can't take reactions, and the only action you can take is a bonus action to emerge from your shell.

When you take the Attack action on your turn, you can replace one of your attacks with an attack of poisonous mucus in either a 15-foot Cone or a 30-foot Line that is 5 feet wide (choose the shape each time). Each creature in that area must make a Dexterity saving throw (DC 8 plus your Constitution modifier and Proficiency Bonus). On a failed save, a creature takes 1d10 poison damage. On a successful save, a creature takes half as much damage. This damage increases by 1d10 when you reach character levels 5 (2d10), 11 (3d10), and 17 (4d10).

You can use this Breath Weapon a number of times equal to your Proficiency Bonus, and you regain all expended uses when you finish a Long Rest.

You have Resistance to Poison Damage.