Arcane Awareness: The abominable whispers will know the exact location of every magic user within 100 feet if them and automatically succeed on all saving throws caused by a spell on the first turn of combat. 

Careful planning: On the first turn of combat the abominable whispers is unable to act. On the second turn however it has advantage on its attack roll. 

Spellcasting. The abominable whispers is a 9-level spellcaster. Its spellcasting ability is wisdom (spell save DC 14, +3 to hit with spell attacks). The abominable whispers has following sorcerer spells prepared:
Cantrips (at will): Mold Earth, Infestation 

1st level (4 slots): Chromatic Orb, Thunderwave

2nd level (4 slots): Detect Thoughts, Acid Arrow

3rd level (3 slots): Dispel Magic, Counterspell

4th level (3 slots): Blight, Vitriolic Sphere

5th level (2 slots): Insect Plague, Enervation

Slam. Melee attack: +5 to hit, reach 70 ft., single target. Hit: 14 (3d6+4) bludgeoning damage. 

Otherworldly Bolt. Ranged magic attack: +3 to hit, reach 30 ft., single target. Hit 15 (3d8+2) necrotic damage. 

As a bonus action the abominable whispers can utter corrupting words to a single creature. The creature must succeed on a DC 14 wisdom saving throw or only be able to act once on their turn. If the creature targeted knows abyssal they have advantage on the saving throw. The abominable whispers has 8 uses per day of this ability.