Bonesnail

Magic Resistance. The snail has advantage on saving throws against spells and other magical effects.

Acidic Trail. The snail leaves around a trail of acidic slime. Any creature that starts its turn within 10 feet of the snail must succeed on a DC 18 Dexterity saving throw or take 21 (6d6) acid damage.

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one creature. Hit: 32 (4d12 + 7) piercing damage plus 21 (6d6) necrotic damage. If the target is a creature other than an undead, it must succeed on a DC 18 Constitution saving throw or be cursed with an undead curse. The cursed creature can't regain hit points, and its hit point maximum decreases by the amount of necrotic damage taken. The curse can be cursed by Greater Restoration spell or after the next long rest.

Acid Spray (Recharge 5–6). The snail exhales a 30-foot cone of acidic slime. Each creature in that area must make a DC 18 Dexterity saving throw, taking 42 (12d6) acid damage on failed save, or half as much damage on a successful one.

As a reaction when targeted with an attack, the snail can hide itself into its shell and impose a disadvantage on all attack rolls until it decides to emerge again (as an action). While in its shell the snail can't attack or move. Acidic trail is still active however.