Urophion

Spider Climb. The urophion can climb difficult surfaces, including along ceilings, without needing to make an ability check.

Multiattack. The urophion makes two Tentacle attacks, uses Reel, and makes two Bite attacks. It can replace one Bite attack with a use of Extract Brain 

Bite. Melee Attack Roll: +7, reach 5 ft. Hit: 17 (3d8 + 4) Piercing damage.

Tentacle. Melee Attack Roll: +7, reach 60 ft. Hit: The target has the Grappled condition (escape DC 15) from one of six tentacles, and the target has the Poisoned condition until the grapple ends.

The tentacle can be damaged, freeing a creature it has Grappled when destroyed (AC 20, HP 10, Immunity to Poison and Psychic damage). Damaging the tentacle deals no damage to the urophion, and a destroyed tentacle regrows at the start of the urophion's next turn.

Reel. The urophion pulls each creature Grappled by it up to 30 feet straight toward it.

Extract Brain. Intelligence Saving Throw: DC 14, one creature within 30 feet of the urophion that is Grappled. Failure: 16 (3d10) Psychic damage, and until the target finishes a Long Rest or dies, its Proficiency Bonus is cumulatively lowered by 1. The urophion gains a cumulative +2 bonus to damage rolls. A creature whose Proficiency Bonus drops to 0 can’t form new thoughts or speak, and it has Disadvantage on attack rolls and d20 Tests. Success: Half damage, and the target's Proficiency Bonus is not reduced. Failure or Success: If this damage reduces the target to 0 Hit Points, the urophion kills it and devours its brain.

Mind Blast. Intelligence Saving Throw: DC 14, each creature in a 30-foot cone. Failure: 21 (6d6) Psychic damage. Success: Half damage.

Mind Reader. The urophion casts Detect Thoughts without spellcasting components, using Wisdom as its spellcasting ability (spell save DC 14):