Alien Mind.

Magic and other features can't determine the corrupter's creature type. If a creature tries to read the corrupter's thoughts, that creature must succeed on a DC 20 Intelligence saving throw or have the stunned condition for 1 minute. The stunned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Legendary Resistance (3/Day).

If the corrupter fails a saving throw, it can choose to succeed instead.

Spider Climb.

The corrupter can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Actions

Multiattack.

The corrupter makes three Whispering Blade attacks and can use Debilitating Touch.

Whispering Blade.

Melee or Ranged Weapon Attack: +12 to hit, reach 5 ft. or range 60 ft., one target. Hit: 10 (1d8 + 6) piercing damage plus 14 (4d6) psychic damage. If the target is a creature, it must succeed on a DC 20 Wisdom saving throw or be unable to speak until the start of the corrupter's next turn.

Debilitating Touch.

The corrupter's squirming tentacle lashes out at a creature the corrupter can see within 30 feet of itself. The target must succeed on a DC 20 Constitution saving throw or have the paralyzed condition until the end of its next turn, after which the target is immune to this Debilitating Touch for 24 hours.

Bonus Actions

Change Shape.

The corrupter magically transforms into any creature that is Small or Medium, while retaining its game statistics (other than its size). This transformation ends if the corrupter is reduced to 0 hit points or uses a bonus action to end it.

Reactions

Uncanny Dodge.

When an attacker the corrupter can see hits the corrupter with an attack, the corrupter halves the attack's damage against it.

Legendary Actions

The otherworldly corrupter can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. The otherworldly corrupter regains spent legendary actions at the start of its turn.

Cunning. The corrupter escapes nonmagical restraints and ends the grappled and restrained conditions on itself, then moves up to its speed without provoking opportunity attacks.
Stab (Costs 2 Actions). The corrupter makes one Whispering Blade attack.
Amorphous Escape (Costs 3 Actions). The corrupter dissolves into a shifting mass of flesh, tentacles, eyes, claws, and mouths. Up to two creatures the corrupter can see within 20 feet of itself must make a DC 20 Intelligence saving throw, taking 18 (4d8) psychic damage on a failed save, or half as much damage on a successful one. This transformation lasts until the end of the corrupter's next turn. While transformed, the corrupter has advantage on attack rolls, attack rolls made against it have disadvantage, and it can move through spaces at least 1 inch wide without squeezing.

Lair Actions

On initiative count 20 (losing initiative ties) while in its lair, the villain can take one of the following lair actions; the villain can't take the same lair action two rounds in a row:

Distraction. A shrill sound fills the lair. Creatures that can hear it must succeed on a DC 15 Constitution saving throw or lose concentration.
Snares. Hidden snares target up to three creatures the villain can see in the lair. Each creature must succeed on a DC 15 Strength saving throw or have the grappled condition (escape DC 15) and the restrained condition until it escapes or the snare holding it is destroyed. Each snare has AC 11, 10 hit points, and immunity to poison and psychic damage.
Traps. Traps target up to two creatures the villain can see in the lair. Each target must succeed on a DC 15 Dexterity saving throw or be struck by a trap. The villain rolls a number of d6s equal to its proficiency bonus, and struck targets take damage equal to the total. The damage is bludgeoning, piercing, slashing, fire, or poison, chosen by the villain when the villain uses this action.

Regional Effects

The region containing a villain's lair is warped by their presence, creating one or more of the following effects:

Corruption Abounds. While within 3 miles of the lair, creatures that aren't members of the villain's organization have disadvantage on ability checks that rely on the Insight, Perception, and Persuasion skills.
Potent Toxins. When members of the villain's organization deal poison damage while within 3 miles of the lair, they ignore the target's resistance to poison damage.

If the villain dies, these effects fade over the course of 1d10 days.

Otherworldly Corrupter