Devotion of the Void. The cultist has advantage on saving throws against being charmed or frightened.

Spellcasting. The cultist is a 13th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 15, +7 to hit with spell attacks). The cultist has following warlock spells prepared:
Cantrips (at will): eldritch blast, mind sliver, toll the dead, chill touch
1st level (4 slots):arms of hadar, mage armor, hellish rebuke, witch bolt
2nd level (3 slots):misty step, crown of madness
3rd level (3 slots):counterspell, fireball, fly
4th level (3 slots):greater invisibility, ice storm
5th level (2 slots):cone of cold, dream
6th level (1 slots):mental prison
7th level (1 slots):dream of the blue veil

Dagger. Melee or Ranged Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 5 (1d4+3) piercing damage.

Aberrant Appendages . As a reaction to being targeted by a spell for the first time in combat, the cultist can nullify the spell and instead channel it into one of the following physical aberrations:

Tentacles. Tentacles burst from the cultist's back, granting them the ability to add an additional tentacle attack each turn.

Tentacle. Melee Attack: +6 to hit, reach 5 ft., one target. Hit: 5 (1d4+3) bludgeoning plus 2 (1d4) necrotic damage.

Gibbering Mouths. Several mouths emerge on the side of the cultist's face and begin to babble incoherently while it can see any creature and isn't incapacitated. Each creature that starts its turn within 20 feet of the mouther and can hear the gibbering must succeed on a DC 16 Wisdom saving throw. On a failure, the creature can't take reactions until the start of its next turn and rolls a d8 to determine what it does during its turn. On a 1 to 4, the creature does nothing. On a 5 or 6, the creature takes no action or bonus action and uses all its movement to move in a randomly determined direction. On a 7 or 8, the creature makes a melee attack against a randomly determined creature within its reach or does nothing if it can't make such an attack.

Bulbous Eyes. Hundreds of bulbous eyes burst through lesions on the skin of the cultist. While the eyes are able to see their surroundings, the cultist gains advantage on all perception checks that involve sight and cannot be flanked.

Spiny Skin. Shards of black crystalline structures burst through the surface of their skin, replacing it with millions of long, needle-like structures. Whenever another creature makes a melee attack on the cultist, they take 6 (1d6+3) piercing damage in retaliation.