Aberrant Ground. The ground in a 10-foot radius around the mouther is doughlike difficult terrain. Each creature that starts its turn in that area must succeed on a DC 10 Strength saving throw or have its speed reduced to 0 until the start of its next turn.

Gibbering. The mouther babbles incoherently while it can see any creature and isn't incapacitated. Each creature that starts its turn within 20 feet of the mouther and can hear the gibbering must succeed on a DC 10 Wisdom saving throw. On a failure, the creature can't take reactions until the start of its next turn and rolls a d8 to determine what it does during its turn. On a 1 to 4, the creature does nothing. On a 5 or 6, the creature takes no action or bonus action and uses all its movement to move in a randomly determined direction. On a 7 or 8, the creature makes a melee attack against a randomly determined creature within its reach or does nothing if it can't make such an attack.

Multiattack. The gibbering mouther makes one bite attack and one lunge attack

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 17 (3d6+2) piercing damage. If the target is Medium or smaller, it must succeed on a DC 12 Strength saving throw or be grappled. 

Lunge. Melee Weapon Attack: +4 to hit, reach 15 ft., one creature. Hit: 9 (2d6+2) bludgeoning damage. If the target is Medium or smaller, it must succeed on a DC 12 Strength saving throw or be pulled to the nearest space adjacent to the mother.  This movement does not provoke attacks of opportunity.

Swallow.  If the mother begins its turn with an enemy grappled, it may use it's action to swallow the creature.  Any creature starting its turn inside the mouther is automatically bitten suffering (3d6+2) piercing damage.  A creature may use its action to try and escape with either a DC13 Strength check or by dealing 15 points of damage within a single round.  A creature that escapes lands prone in an unoccupied space adjacent to the mouther.