I decided to give it an "emotion" keyed to them like the Du'ulora (anger), Hashalaq (desire), and Tsucora (fear), and decided that "insecurity" fit in an odd way. That inner voice that you hear in your head that whispers deprecating thoughts. I thought that was a good fit for another kind of nightmare. Not the kind that terrifies you, but the kind that messes you up emotionally, makes you question your worth. Seems like something the quori would have in their ranks.
Usvapna quori serve the hashalaqs as their spies and assassins. Also known as "dream masters", the usvapna revel in twisting the dreams of mortals against themselves, turning hope to despair and triumph into anguish. They can hear the secret doubts of mortals, the voices in their minds that foment hesitation and insecurity. Usvapna quori hear this voice in their own minds, but unlike mortals their inner voices are in complete accord with themselves, allowing the dream masters to split their mental attention in two with ease.
Large fiend (quori), lawful evil
Armor Class 17 (mage armor)
Hit Points 114 (12d10 + 48)
Speed fly 40 ft. (hover)
Saving Throws Wis +8, Cha +8
Skills Arcana +7, Insight +8, Intimidate +8, Perception +8, Persuasion +8, Stealth +8
Damage Resistances acid, cold, fire; bludgeoning, piercing, and slashing damage from nonmagical attacks
Condition Immunities charm, frightened
Senses darkvision 60 ft., passive Perception 18
Languages Common, Quor, Reidran, telepathy 120 ft.
Challenge 10 (5,900 XP)
Magic Resistance. The usvapna has advantage on saving throws against spells and other magical effects.
Nightmare Awareness. Magical darkness doesn't impede the quori's darkvision, and magic can't put the quori to sleep.
Innate Spellcasting (Psionic). The usvapna's innate spellcasting ability is Charisma (spell save DC 15). It can innately cast the following spells, requiring no components:
At will: enthrall (the distracting word are spoken telepathically), detect thoughts, mage armor (the armor is invisible), magic missile (one target, 3 invisible missiles), teleport (on Dal Quor only)
1/day each: cure wounds (self only; 6th level; 35 (7d8+4) healing), inflict wounds (6th level; 33 (6d10) psychic damage), modify memory
Mind Schism. The usvapana quori may cast any spell it knows as a bonus action if the spell normally has a casting time of 1 action. It is not limited to cantrips if it casts a second spell on its turn.
Invade Dreams. See the Tuscora quori for the full description of this ability, though the Wisdom saving throw DC is 15.
Quori Possession (Recharge after a Long Rest). See the Tuscora quori for the full description of this ability.
Multiattack. The usvapna makes four pincer attacks.
Pincer. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: 15 (2d10 + 4) bludgeoning damage and the target is grappled (escape DC 15). The usvapna has four pincers, each of which can grapple only one target.
_If a creature is hit by two pincer attacks in one turn, the creature must succeed on a DC 15 Charisma save. On a failure, the creature is incapacitated for 1 round while its mind is filled with doubt and it hears thoughts of self-loathing. A creature with Intelligence 2 or less is unaffected.