Alkilith

The alkilith can move through a space as narrow as 1 inch wide without squeezing.

While the alkilith is motionless, it is indistinguishable from an ordinary slime or fungus.

Any creature that isn't a demon that starts its turn within 30 feet of the alkilith must succeed on a DC 18 Wisdom saving throw, or it hears a faint buzzing in its head for a moment and has disadvantage on its next attack roll, saving throw, or ability check.
If the saving throw against Foment Madness fails by 5 or more, the creature is instead subjected to the Confusion spell for 1 minute (no Concentration required by the alkilith). While under the effect of that confusion, the creature is immune to Foment Madness.

The alkilith has advantage on saving throws against spells and other magical effects.

The alkilith makes three tentacle attacks.

Melee Weapon Attack +8 to hit, reach 15 ft., one target. 18 (4d6 + 4) Acid damage.