Urophion

False Appearance. While the roper remains motionless, it is indistinguishable from a normal cave formation, such as a stalagmite.

Innate Spellcasting (Psionics). The urophion's innate spellcasting ability is Intelligence (spell save DC 14). It can innately cast the following spells, requiring no components. 

At will: detect thoughts, Hold person

1/day:dominate beast, dominate person (humanoid), dominate monster Locate creature

Magic Resistance. The urophion has advantage on saving throws against spells and other magical effects.

Grasping Tendrils. The roper can have up to six tendrils at a time. Each tendril can be attacked (AC 20; 10 hit points; immunity to poison and psychic damage). Destroying a tendril deals no damage to the roper, which can extrude a replacement tendril on its next turn. A tendril can also be broken if a creature takes an action and succeeds on a DC 15 Strength check against it.

Spider Climb. The roper can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Mind Blast Immunity. The urophion is immune to the Mind Blast of mind flayers and similar creatures.

Multiattack. The roper makes four attacks with its tendrils, uses Reel, and makes one attack with its bite.

Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 22 (4d8 + 4) piercing damage.

Tendril. Melee Weapon Attack: +7 to hit, reach 50 ft., one creature. Hit: The target is grappled (escape DC 15). Until the grapple ends, the target is restrained and has disadvantage on Strength checks and Strength saving throws, and the roper can't use the same tendril on another target.

Mind Blast (Recharge 5–6). The mind flayer magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 15 Intelligence saving throw or take 22 (4d8 + 4) psychic damage and be stunned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Extract Brain. One Large or smaller creature grappled by the urophion must succeed on a DC 15 Strength (Athletics) or Dexterity (Acrobatics) check (target's choice). On a failed save, the urophion makes a bite attack against the creature with advantage. If the attack hits, the creature takes 55 (10d10) piercing damage instead of the normal damage for the attack. If this damage reduces the target to 0 hit points, the urophion kills the target by extracting and devouring its brain.

Reel. The roper pulls each creature grappled by it up to 25 feet straight toward it.