My PCs have conquered the bad guys in “A Darkness Gathering” and now head off to the North in “Masters of Eternal Night“.  This adventure has a lot of mind flayers so I created slightly different versions to make any battles a bit more interesting. Crgent is found in area 25 along with his oculus bodyguard – waiting to ambush the PCs. I made all the ancient mind flayers dress like the 1990s Planescape mind flayers with their “top hat” and their priestly/tuxedo type robes. I also started incorporating their levitate innate spell-like ability as part of their move action.  Preserves this very mind flayer trait without adding a spell block.

It also goes to show that a little modification a “low CR monster” – at least when compared to a level 17 party – works quite well as a “normal” threat.

Medium aberration (mind flayer), lawful evil
Armor Class 18 (bone ring)
Hit Points 117 (13d8 + 13)
Speed 30 ft., fly 20 ft (levitate only)

11 (+0) 12 (+1) 17 (+3) 19 (+4) 17 (+3) 17 (+3)

Saving Throws INT +8, WIS +7, CHA +7, +1 to all others (see bone ring).
Skills Arcana+7, Deception+6, Insight+6, Perception+6, Persuasion+6, Stealth+4
Damage Vulnerabilities lightning (see Dissipating Lake Rune)
Senses Darkvision120 ft., passive Perception 16
Languages Deep Speech, Telepathy 120 ft. (can detect thoughts as part of this trait), Undercommon
Challenge 7 (2,900 XP)

Dissipating Lake Rune.This rune is etched in the mind of Crgent and gives him resistance to lightning damage.
Magic Resistance.Crgent has advantage on saving throws against spells and other magical effects.
Innate Spellcasting (Psionics).Crgent’s innate spellcasting ability is Intelligence (spell save DC 15). It can innately cast the following spells, requiring no components:
At will: detect thoughts, levitate
1/day each: dominate monsterplane shift (self only)

Multiattack. Crgent attacks once with his tentacles and once with his threaded cane.
Tentacles. Melee Weapon Attack: +7 to hit, reach 5 ft., one creature.
Hit: 15 (2d10 + 4) psychic damage. If the target is Medium or smaller, it is grappled (escape DC 15) and must succeed on a DC 15 Intelligence saving throw or be stunned until this grapple ends.

Threaded Cane.Melee Weapon Attack: +7 to hit, reach 15 ft., one creature.
Hit:28 (5d10+1) slashing damage if used against a target more than 5 ft away or 7 (2d6+1) bludgeoning damage if used within 5 feet.

Extract Brain.Melee Weapon Attack: +7 to hit, reach 5 ft., one incapacitated humanoid grappled by the mind flayer.
Hit: The target takes 55 (10d10) piercing damage. If this damage reduces the target to 0 hit points, the mind flayer kills the target by extracting and devouring its brain.

Mind Blast (Recharge 5–6). Crgent psionically emits 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.

Bone ring (AC 18 and +1 to saving throws for mind flayers, ring of protection for other creatures), brainmate lobe, dissipating lake caryl rune, threaded cane (a whip cane).