Unhallowed Choir

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.
Chant. The Unhallowed Choir chants in a foul tongue known only to it, while it can see any creature and isn't incapacitated. Each creature that starts its turn within 20 feet of the Unhallowed Choir and can hear the chanting must succeed on a DC 15 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, joining in the song under their breath. 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 so toward itself if their is no other target. 

Multiattack. The gibbering mouther makes four bite attack and, if it can, uses its Enveloping Lunge.

Bite.. Melee Weapon Attack: +2 to hit, reach 5 ft., one creature. Hit: 17 (5d6) piercing damage. If the target is Medium or smaller, it must succeed on a DC 10 Strength saving throw or be knocked prone. If the target is killed by this damage, it is absorbed into the Unhallowed Choir.

Enveloping Lunge.  one target creature that 0 I One creature that is Large or smaller and is within, it must succeed a DC 15 Dexterity saving throw or be ingested. only one creature may be ingested at a time. Ingested creatures take 3d6 poison damage each turn they within the Unhallowed Choir. Ingested creatures take half of any damage dealt to the Unhallowed Choir.

 Assimilate: When the Unhallowed Choir absorbs a dead creature, it gains maximum health points equal to that of the consumed's original maximum.

Adapt: The Unhallowed Choir gains resistance to a type of damage dealt to it