Running a campaign that involves A LOT of mind flayers requires doing some interesting things fight to fight with the standard mind flayer. Stealing an idea from 13th Age here are “nasty specials” that can be thrown onto a mind flayer to make them unique and deadlier. These were mostly ripped from 4e and converted.
MIND FLAYER NASTY SPECIALS
Pick one of these to spice up the many mind flayer encounters. Either replace traits that have the same name or add these ones. Anything in  are optional extras for each trait.
Special Equipment: Resin Battle Armor. This enhanced dampsuit provides the following benefits to the mind flayer – +6 AC, Damage Reduction of 10, Strength 20 (15 original), extends the reach of its tentacles by 5 feet to 15 feet, adding blades to the end of each tentacle allowing the mind flayer to switch damage types to slashing for is tentacle attacks if it chooses and adds the multiattack action. It also provides environmental protection to the mind flayer allowing it to ignore weather extremes that are natural in nature.
Stalk the Senseless. The mind flayer is invisible to stunned or dazed creatures (and/or creatures that have suffered psychic damage in the last round). If adding this trait mind blast causes the daze state for 1 minute even on a failed saving throw. A dazed creature has disadvantage on attack rolls and Intelligence saving throws.
Mind Leech.Whenever the mind flayer starts its turn with a creature grabbed. the mind flayer can make a saving throw or an extra recharge roll.
Aura: Psychic Static. While the mind flayer is not bloodied, enemies in the 10 foot aura take a –2 penalty to AC, Intelligence and Wisdom saving throws.
Multiattack. The mind flayer makes two [four or six] tentacle attacks.
Brutal Tentacles. Melee Weapon Attack:+11 to hit, reach 15 ft., one creature.
Hit: 45 (4d10 + 5) psychic damage. If the target is Large or smaller, it is grappled(escape DC 14) and must succeed on a DC 17 Intelligence saving throw or be stunneduntil this grapple ends.
*Replaces Tentacle attack.
Mind Lance (Recharge 5–6).The ulitharid magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 17 Intelligence saving throw or take 50 (5d8 + 5) psychic damage and be stunnedfor 1 minute. A target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
*Replaces Mind Blast.
Enslave (Recharge on a Successful Save). The mind flayer chooses a target it can sense within 50 feet and attempts to dominate it. The target must make a DC 17 Wisdom saving throw or become dominated for 1 minute (as the dominate personspell). While dominated, the target is immune to the mind blast power of mind flayers and has advantage to its Wisdom saving throw while within 50 feet of the controlling mind flayer. The mind flayer can use enslave on only one creature at a time.
Tentacle Projection(Recharge 5–6). The mind flayer selects a creature it can see within 50 feet, tentacles explode from the ground grappling the target if it is medium or smaller (Escape DC 15) on a failed DC 17 Dexterity saving throw. A creature that fails its saving throw takes 25 (4d10 + 5) psychic damage and is stunned until the grapple ends. While the mind flayer has a target grappled and stunned by this power. it can make its extract brain attack as a ranged attack. [The mind flayer may make this attack as a reaction the first time a creature is grappled, or a bonus action thereafter].
Psychic Assault. Melee Weapon Attack: +7 to hit, reach 5 ft., one creature.
Hit: 15 (2d10 + 4) psychic damage. If the target is a Medium or smaller humanoid, it is grappled (escape DC 15) and must succeed on a DC 15 Intelligence saving throw or become dominated (as the dominate person spell) for 1 minute. A creature that makes its saving throw has a disadvantage on Intelligence saving throws until the next minute.
Implant Delusions. Each time a creature takes psychic damage from the mind flayer it must make a DC 17 Wisdom saving throw or suffer delusions. The target acts in such a distracting and erratic fashion that allies who can see or hear it take a -2 penalty to attack rolls and a -2 penalty to all AC and saving throws. This effect ends after 1 minute of not taking any psychic damage.
Own Worst Enemy (Recharge when first bloodied). Each enemy in an area 10-foot sphere within 50 feet uses one of its ranged or melee attack actions of the mind flayer’s choice against itself as a free action.
Agonizing Feedback (Recharge 5-6).The mind flayer creates three balls of psychic fire and hurls them at targets within 120 feet. The mind flayer may hurl them at one target or several. Use the mind flayers tentacle to-hit attack value for each ball. On a hit, the target takes 7 (2d6) fire and 7 (2d6) psychic damage, and the target is dazed. If the target is already dazed, it instead becomes stunned for 1 minute (a DC 17 Intelligence save ends both conditions). The mind flayer teleports 25 feet after the attack.
Bore into 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 may do one of the following as part of the attack:
- Devour Brain. The mind flayer devours the target’s brain. The target is killed instantly, and the mind flayer regains 55 hit points.
- Create Thrall. The target is dominated (no save, otherwise as the dominate personspell) and regains enough hit points to restore it to its bloodied value. As a thrall, the target is immune to the mind blast power of mind flayers and gains a +5 bonus to its Wisdom saving throw while within 50 feet of the controlling mind flayer. The target is no longer dominated and no longer a thrall once the controlling mind flayer dies.
*Replaces Extract Brain.
Alter Reality. If the mind flayer would be struck by an attack it may use its reaction to teleport up to 10 feet away. The triggering attack misses.
Forceful Submission. The mind flayer may force an enemy that succeeds a saving throw to make that saving throw again at disadvantage.
Cradle of the Elder Brain. If the mind flayer would be struck by an attack it may use its reaction to teleport up to 100 feet away. The triggering attack misses.
Interpose Thrall. If the mind flayer would be struck by an attack it instead is redirected to an adjacent thrall.
The mind flayer can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The mind flayer regains spent legendary actions at the start of its turn.
- Psion. The mind flayer manifests a psionic power.
- Psychic Warrior. The mind flayer makes a tentacle attack.
Forceful Submission (Costs 1 or 2 Actions).The mind flayer may force an enemy that succeeds a saving throw to make that saving throw again at disadvantage. This legendary action can be used as a rider on the mind flayer’s Mind Blast but this costs 2 actions.
- You grant advantage.
- You can take only one action and lose reactions and cannot takes opportunity attacks.
- You can’t flank an enemy.