Mindrider

A failed Mind Flayer experiment, the result of forcing a Drider to undergo ceremorphosis.
The creature still vaguely retains its drow-torso-on-spider-body shape, and the chitinous exoskeleton that makes the Drider so well-armored, but a thick layer of rubbery squidskin grows over the chitin, sloughing loosely as the creature moves. Three huge tongue-tentacles extend nearly 6' from its mouth, which loses its lower jaw completely as well as the ability to speak in any intelligible way; the process also destroys its intellect so it's not like it has much to say anyway. It is a very good listener, however, able to sense the presence of any being with an intelligence greater than 7 within 90' and to eavesdrop on any conversation in that area without being detected, if it is hiding, which it isn't bad at as it can stlill cling to any surface horizontally or vertically including inverted. It prefers to travel on the ceiling rather than the floor, and the only hint of its presence might be a glop or two of saliva from time to time if it fails a stealth check.
It has limited but effective psionic powers, using it's telepathy an the attack mode Dominate to injure and charm victims into passively allowing it to snuggle its tentacles over their heads and then, if it can, to pull their brain out and eat it. It's not nearly as good at this as a real mind flayer, but what it lacks in style it nevertheless makes up for in effectiveness once it has its tentacles in a victim's head.
Though not as hard to hit as a Drider, as it lacks the grace and fey advantages of the real thing, its weird hybrid body can soak up a lot of damage before expiring, and when it dies it sets off a psychic shriek that can be 'heard' by any telepathically aware creature within 300'. More powerful telepaths like real Mind Flayers and Elder Brains are aware of Mindriders on their network and generally tolerate them if they were the ones who made them, but quickly search out and kill them if they're interlopers from outside their colony.

Mindrider

Large aberration, lawful evil

Armor Class 16

Hit Points 13d10+65

Speed 30ft Climb: 30ft

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STR

14 +2

DEX

14 +2

CON

20 +5

INT

7 -2

WIS

14 +2

CHA

15 +2

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Saving Throws CON +11, DEX +8

Skills (Proficiency +3)
Perception +5, Stealth +8
Spider Climb. The Mindrider can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.
Sunlight Sensitivity. While in sunlight, the Mindrider has disadvantage on attack rolls, as well as on Wisdom (Perception) checks that rely on sight.
Web Walker. The Mindrider ignores movement restrictions caused by webbing.

Damage Resistances Resistant to normal slashing, piercing and bludgeoning. Magic Resistance. The Mindrider has advantage on saving throws against spells and other magical effects.

Senses Telepathy 90', Darkvision 120'. Passive Perception 14

Languages Undercommon, Telepathy 90'

Challenge 6

Psionics

PPP: 27, recharge 3 per hour

Attack Modes:

Charisma (+2) Dominate 3d8 + charm 1 min, Save DC15

Defense Modes

Idealized Self. Advantage and +10 total on saves against Charm effects

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Actions

Tentacles. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 2d10+2 psychic damage. If the target is Medium or smaller, it is grappled (escape DC 15) and must succeed on a DC 7 Intelligence saving throw or be stunned until this grapple ends.
Extract Brain. Melee Weapon Attack: +5 to hit, reach one charmed or stunned humanoid grappled by the mindrider Hit: The target takes 10d10 piercing damage. If this damage reduces the target to 0 hit points, the mindrider kills the target by extracting and devouring its brain.
Psionic Attack: Dominate (recharge 5-6). CHA save DC15 or take 3d8 psychic damage and charm 1 min. The nature of the charm is primarily to become docile and passively accept the grapple of the Mindrider.
Psionic discipline: Telepathy. The Mindrider is passively aware of the minds of creatures with an intelligence higher than its own within 90', and can listen to any tepeathic communication taking place in that range. However it can only haltingly communicate in halting undercommon, telepathically as its mouth is no longer able to form normal language sounds.

The Mindrider is the unholy result of a Drider undergoing Mind Flayer Ceremorphosis. Considered a failed experiment by the Illithid, the process burns out most of their sentient intelligence, replaced by an aberrant charm and cunning. While retaining the basic shape of a drider, with a slobbering, 3-tentacled torso where the Drow upper body had been, all its Fey nature is gone and it is more Illithid than Drow. These creatures are considered a slay-on-sight abomination to the worshippers of Lolth.
What they lose in intelligence, they make up for in toughness, retaining their exoskeletal chitin but gaining a leathery, rubbery squidflesh over it that grants resistance to normal slashing, piercing and bludeoning as well as normal force damage from explosions and the like. This comes complete with suckers that retain their ability to climb vertically and even upside-down on normal surfaces.

Underdark