Parasite. The mouth can merge with the surface of any creature or object large enough to contain it; if the creature or object moves, the mouth moves with it. As part of its movement, the mouth can either submerge, gaining full cover and advantage on its Hide checks, but being unable to see, speak, or attack, or emerge, reducing cover to 3/4 (+5 AC and Dex save), but allowing it to see, speak, and attack.

Immovable. The mouth is immune to effects that would move it, though its container can move.

Disease. If melded with a living creature, the mouth is considered a disease; any effect that cures disease will kill the mouth unless the mouth succeeds at a Constitution save. It is possible to cut the mouth out; a successful Medicine check (DC 15) allows damaging a melded mouth, but any damage done to the mouth is also done to the host.

Gibber. The mouth babbles incoherently. Until its next turn, each creature that starts its turn within 20 feet of the mouth 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.

Tongue. Melee Weapon Attack: +2 to hit, reach 5', one creature. Hit: 3 (1d6) acid damage, and the target must make a DC 10 Strength save or be Grappled (DC 10 to escape). A grappled creature is subject to Bite on subsequent turns.

Spit. +2 to hit, range 30/60, one creature. Hit: 3(1d6) acid damage.

Bite. Melee Weapon Attack: +2 to hit, reach 0 ft., one creature. Hit: 3 (1d6) piercing damage plus 3 (1d6) acid damage, and the mouth may use its Infest attack.

Infest (once per day). A bitten target or an immediately adjacent object must make a Constitution save (DC 13) or be infected by a mouth, which remains in a larval state for 24 hours before being able to emerge. The target's maximum hit points drop by 5 (ends when the mouth is killed; a larval mouth has 1 hp and automatically fails to resist disease cures). If a creature's maximum hp drop to zero, they die and a new gibbering mouther (infester variant, see below) grows from the corpse. A mouth that has used its infest attack immediately submerges and remains that way until its infest recharges, or someone attempts to cure it.

Reactive Meld. If the mouth is missed by an attack, succeeds at a Dexterity save, or succeeds at a save against an area effect, it may meld with the surface it is on, gaining full cover against that effect.