Abyssal Rift. If the alkilith surrounds a door, window, or similar opening continuously for 6d6 days, the opening becomes a permanent portal to a random layer of the Abyss.

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

False Appearance. If the alkilith is motionless at the start of combat, it has advantage on its initiative roll. Moreover, if a creature hasn't observed the alkilith move or act, that creature must succeed on a DC 18 Intelligence (Investigation) check to discern that the alkilith isn't ordinary slime or fungus.

Foment Confusion. 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 Confusion 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 Confusion.

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

Spider Climb. The alkilith can climb difficult surfaces, such as upside down on ceilings, without making an ability check.

Unusual Nature. The alkilith doesn't require air, food, drink, or sleep.