Magic Resistance. The mask horror has Advantage on saving throws against spells and other magical effects.

Special Reaction. The mask horror can take on extra reaction each round. It can only use this reaction to use Adaptation.

Teleport Blocking Aura. Any non-construct creature that attempts to teleport from or to a space with 10 feet of mask horror takes 7 (2d6) Psychic damage and the teleport fails to move them.

Immutable Form. The mask horror can’t shape-shift.

Anti-Magic Weaponry. When the mask horror hits with a attack against a creature that is concentrating on a spell it can chose to not trigger a save on that damage but instead deal critical damage instead. It also Autocritical against magical constructs when attacking. 

Multiattack. The mask horror makes two attacks using Arcane Claws. It can also replace one of its attacks with its Unraveling Ray action or Dispel Magic action if available.

Arcane Claws. Melee Attack Roll: +8, reach 5 ft. Hit: 7 (1d8 + 5) Slashing damage plus 9 (2d8) points of damage of a type it's immune to and the target speed becomes 0 until the end of this turn. Hit or Miss: This attack ignores spells or magic items that increase the AC of a creature such as the shield spell or a ring of protection.

Unraveling Ray. Dexterity Saving Throw: DC 15, one creature with 60 feet of the mask horror that it can see (Creatures with the spellcasting feature have disadvantage on the save) Failure: the creature takes 26 (4d12) Force damage and if the creature can cast a spell it loses a spell slot of at least 1st level or a use of a spell with limited uses. It then takes 3 points of force damage equal to the spell slot lost or the level of the spell lost because of this ability. Success: Half damage only.

Dispel Magic (Recharge 5–6). The mask horror casts Dispel Magic (level 4 version), requiring no spell components and using Intelligence as the spellcasting ability.

Teleport. The Mask horror teleports up to 40 feet to an unoccupied space.

Second Hand Attack. The mask horror makes on Arcane Claw attack.

Adaptation. Trigger: The mask horror takes damage that isn't a damage it is Resistance to. Response: The mask horror gains Immunity to that damage type until it finfishes a Short rest or until it has used this reaction again (If it's takes two or more damage types at once it gains immunity to all of them).

Pursuit. Trigger: Another creature the mask horor can see ends its move within 40 feet of the mask horror. Response: The mask horror uses Teleport, but its destination space must be within 5 feet of the triggering creature.