Slayer's Puppet (1/Day). The mindkiller casts Dominate Person, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 17).
Multiattack. The mindkiller makes two Nightmare Whisper attacks.
Nightmare Whisper. Melee or Ranged Attack Roll: +9, reach 5 ft. or range 30 ft. Hit:27 (5d8 + 5) Psychic damage, and the target has the Frightened condition until the start of the mindkiller's next turn. If the target is already Frightened, it instead takes an extra 10 (3d6) Psychic damage.
Primal Fear (1/Day). Wisdom Saving Throw: DC 17, each creature in a 30-foot-radius Sphere centered on a point the mindkiller can see within 120 feet. Failure: 35 (10d6) Psychic damage, and the target has the Frightened condition until the start of the mindkiller's next turn. While Frightened in this way, the target can do only one of the following on each of its turns: move, take an action, or take a Bonus Action. Success: Half damage only.
Mindkillers are the assassins of the Dreaming Dark, sent to terrify their foes before striking to kill. Mindkillers force foes to attack their own allies, causing physical and emotional devastation to grow the power of the Dreaming Dark.