Regurgitate Aeons. (Once per short rest)  As a bonus action, The Snail God unleashes years of old from its wretched maw. 60 foot line, 10 feet wide. Dex Save. DC 20 15d6 acid damage. Half on success. All creatures who failed the save must subtract 5 from their AC until they take an action to wipe off the acid.

Swallow. Melee Weapon Attack: +12 to hit, reach 5', one Medium or smaller creature. Hit: (3d12 + 12) bludgeoning damage, and if the target is a creature, it is grappled (escape action DC 20). If the target is still grappled at the start of the snail's next turn, it is swallowed and the grapple ends. The snail cannot swallow a creature while it has another living creature swallowed.
While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the snail, and it takes (20d4) acid damage at the start of each of the snail's turns.
If the snail takes 50 damage or more on a single turn from a creature inside it, the snail must succeed on a DC 16 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 5' of the snail.

Turn Back The Clock. As an action, The Snail God can expend a spell slot to regain hit points equal to a number of d12s equal to the level of the spell slot expended.

Your Time Is Up. When time is stopped, Achatinoidea can spend an action and expend a spell slot to set up an attack that cannot miss. The attack hits as soon as time resumes. The attack deals a number of d12s of force damage equal to the level of spell slot used for this action.

Achatinoidea has 3 legendary actions which recharge at the start of its turn. It can choose from the following options:

Swallow.

Spell Shell (2 Actions) Cast a cantrip, gain + 2 AC until the start of your next turn

Slime Time (3 actions).  Achatinoidea recharges his use of Regurgitate Aeons.