The cultist makes two Tentacle Lash attacks. It can replace any attack with a use of Mind Rot.
Tentacle Lash
Melee Attack+7, reach 10 ft. 7 () 🗡️ Slashing damage plus 14 () 🧠 Psychic damage. If the target is a Large or smaller creature, it has the condition (escape DC 14) from one of two tentacles, and it has the condition until the grapple ends.
Mind Rot
WIS SaveDC 15, one creature the cultist can see within 90 feet. Failure: 27 () 🧠 Psychic damage, and the target has the condition until the start of the cultist's next turn. Success: Half damage only.