Multiattack. The cultist makes two Tentacle Lash attacks. It can replace any attack with a use of Mind Rot.
Spellcasting. The cultist casts one of the following spells, using Wisdom as the spellcasting ability (spell save DC 15):
At will: Detect Thoughts, Minor IllusionTentacle Lash. Melee Attack Roll: +7, reach 10 ft. Hit: 7 (1d6 + 4) Slashing damage plus 14 (4d6) Psychic damage. If the target is a Large or smaller creature, it has the Grappled condition (escape DC 14) from one of two tentacles, and it has the Restrained condition until the grapple ends.
Mind Rot. Wisdom Saving Throw: DC 15, one creature the cultist can see within 90 feet. Failure: 27 (6d8) Psychic damage, and the target has the Poisoned condition until the start of the cultist's next turn. Success: Half damage only.
Counterspell (2/Day). The cultist casts Counterspell in response to that spell's trigger, using the same spellcasting ability as Spellcasting.