This is part of a series! Go here to see the other entries.
Mind flayers are one of those monsters that are so iconic to D&D they’re not covered by the D20 license. They’ve been in the game since AD&D 1st Edition, and are present in both the Monster Manual and the Monster Vault.
Mind flayers, or illithids, are tentacle-faced humanoids who possess incredible psychic powers and use them to feed on sapient brains. They came from the Far Realm long ago. Before they arrived in the world, they had a great dimension-spanning empire whose experiments were responsible for the creation of the Gith peoples as we know them today. That empire was torn down when their slaves rebelled, and the illithids of today descend from its survivors.
Some versions of mind flayer lore assert that they come from a distant future, having travelled back in time to ensure their empire rises and to prevent its fall. A necessary step in this plan would be to turn the world into another region of the Far Realm.
Your typical mind flayer is a solitary, megalomaniacal villain bent on total world domination, who surrounds itself with large numbers of thralls to act as its hands and eyes to enact is elaborate plans on whatever society they’re targetting. That doesn’t mean the illithid is a coward! Should it see the need to get into a fight personally, it will do so without hesitation. There’s no limit to the number of thralls a single illithid may control - the process of creating them involves brain surgery, not active psychic domination. So those organizations may be quite large, and composed of a mix of thralls and willing (or ignorant) collaborators.
Sometimes, several mind flayers with similar goals will work together, though you probably can’t call their relationship “friendly”. In the deep Underdark you can find large illithid cities occupying huge caverns, where many of these beings gather and surround themselves with veritable armies of thralls. These cities are ruled by Elder Brains, huge masses of illithid neural tissue that live inside briny vats.
Mind flayers start their lives as tiny tentacled tadpoles, who upon reaching a certain level of maturity break into some poor sapient’s skull, devour its brain, and take over their bodies to mutate them into their adult form. In illithid cities, these tadpoles live in the Elder Brain brine pool until someone gives them a body.
Some versions of mind flayer lore mention that illithids who die in these cities have their minds assimilated by the Elder Brain. They consider this to be the perfect afterlife, believing their conscience lives on inside the Brain as part of a collective. The truth is that while the process does make the Elder Brain more powerful, it erases the conscience of the individuals joined to it.
5e tied all illithids a lot more closely to the Elder Brains, making them practically into a hive mind, but this isn’t true here. Individual, independent illithids are exactly that. They don’t need exceptional reasons to reject life in an Underdark city - maybe their plans for world domination are just incompatible (“I’m going to be the boss, not you”). This means that it’s perfectly possible to fight mind flayers in the surface without this being necessarily tied to some Underdark conspiracy.
The Monster Manual gives us 2 mind flayer stat blocks, and the Vault gives us 3. All are Medium Aberrant Humanoids with Darkvision and a land speed of 7. Every mind flayer can use a mind blast, grapple with its face tentacles, and do creepy things to people’s brains, though the exact effects vary.
All Mind Flayers understand Deep Speech, and can communicate via telepathy regardless of language barriers.
Mind Flayer Infiltrator (MM)
This is a Level 14 Lurker with 107 HP and trained Perception, as well as a host of other thiefy skills.
It it has to fight, the infiltrator will probably open up with its Mind Blast (Close Blast 5 vs. Will; recharge 6), which does psychic damage and dazes on a hit (save ends). Mind flayers and thralls are immune to this power, and the infiltrator itself is invisible to any dazed or stunned enemies (Stalk the Senseless).
It will then attack the squishiest target with its tentacles, which grab the target on a hit. After grabbing a victim, it will proceed with Bore Into Brain (Melee 1 vs. Fortitude), something which it can also use against stunned targets without grabbing them first. This does more damage than the basic attack, and if it happens to reduce the target’s HP to 0, the illithid eats its brain and kills it instantly. No death saves for you!
As lurkers, they probably give preference to targets made vulnerable by the Mind blast, and rely on their buddies and thralls to handle anyone who wasn’t affected. MM grab rules use either the Fort or Reflex as escape DCs, so you’re looking at a DC of 25 or 27 to get away from the infiltrator before it begins eating your brain.
Mind Flayer Thrall Master (MV)
This one is a Level 14 Elite Controller (Leader) with 260 HP. Its trained skills are Arcana and Insight, so it’s more of a scholar than a hands-on type.
That doesn’t mean its defenseless! The thrall master fights in a way that’s somewhat similar to the MM’s Infiltrator, but with stronger psychic powers. Its Mind Blast (Close Blast 5 vs. Will; Recharge 5-6) works similarly but on a critical hit it dominates instead of dazing (save ends).
The tentacles still grab on a hit, but they also hit dazed or stunned targets automatically. The escape DC is 21, and while the thrall master can only grab one person at a time, it can explicitly keep whipping other fools with its tentacles after that.
Against the grabbed victim, the mind flayer can use Manipulate Brain (vs. Fortitude), which does a big chunk of physical damage and stuns the target until it’s no longer grabbed! If this reduces the victim to 0 HP, the thrall master has two choices: it can either devour the brain, killing the victim and regaining 20 HP, or it can perform a field-expedient thrall surgery. This restores the victim to 1 HP and dominates it until the thrall master is slain.
Since the dominated condition also leaves the victim dazed, this results in a 1-HP, zombie-like thrall, whose main defence is the fact that its companions might not want to make it roll death saves. I imagine the illithid can do further work out of combat to make these thralls act more normally.
Mind Flayer Mastermind (MM)
This is essentially a more powerful variant of the Monster Vault’s Thrall Master. It’s a Level 18 Elite Controller with 324 HP and training in Perception, Arcana, Insight, Bluff and Intimidate.
It can use its tentacles, Mind Blast and Bore Into Brain abilities in much the same way the thrall master can. Aside from the damage bug, the main difference here is that thrall surgery restores the victim to half its maximum HP, not just 1.
The mastermind can directly Enslave a creature (Ranged 10 vs. Will), an attack which does no damage but dominates (save ends). Dominated creatures become immune to the flayer’s mind blast and gain a +5 bonus to Will while within 10 squares of it. The victims it makes into thralls also gain this benefit, by the way. Only one creature can be enslaved at a time, but as usual it can have as many thralls as it can make.
The mastermind can also plant an Illusion of Pain in its victims (Area Burst 1 within 10 vs. Will; Enemies only; recharge 5-6) which does both immediate and ongoing psychic damage and immobilizes (save ends).
When attacked, it can use Craddle of the Elder Brain (recharge 5-6) as an interrupt, teleporting up to 20 squares. If it’s targeted by a melee attack, it can instead Interpose Thrall, an interrupt which redirects the attack to a nearby thrall. That’s definitely going to be its preferred option if it has thralls nearby, with the teleport working as a tool of escape.
Mind Flayer Unseen (MV)
A more powerful version of the Infiltrator, this one is a Level 18 Lurker with 126 HP. It’s almost identical aside from the fixed damage, which makes its Extract Brain attack truly frightening.
Instead of being invisible to dazed characters, the Unseen has a Mental Cloak power which, as a move action, allows it to teleport its speed and become invisible for a turn. It recharges whenever the Unseen is hit by an attack.
Concordant Mind Flayer (MV)
This Level 19 Controller (Leader) also works more or less like the Thrall Master. It has 174 HP.
Its attacks are the usual trifecta of Mind Blast, tentacles and brain manipulation, which in this case is named Enthrall Brain and only gives the option of turning victims reduced to 0 HP into thralls. Like the Thrall Master, its mind blast dominates on a critial hit.
The Concordant Mind Flayer’s exclusive trick is Shared Pain, a reaction that triggers when an ally within 5 squares takes damage from an attack. The illithid can make one of its dominated victims in a Close Burst 5 take 20 psychic damage which bypasses all of its resistances. This is sure to kill someone that’s been made into a thrall during the fight, since they’ll only have 1 HP. It doesn’t prevent damage to the ally - it’s more of a hostage situation. “Attack my friends and your friends will suffer!”
Sample Encounters and Final Impressions
Mind flayers typically consider themselves superior to everyone else. Anyone found working with them is probably a thrall or underling. I’d consider thralls created outside of combat to be able to act normally instead of being considered dazed.
It’s possible for there to be a more congenial illithid that lives openly as a scholar and gentlesquid in a community ruled by another species. The most likely scenario in this case is that it lives and works alongside drow or duergar - congenial doesn’t mean good.
This gives us the two sample encounters from the MM:
Level 14: 1 mind flayer infiltrator, 1 drider fanglord, 1 drow blademaster, 2 drow warriors. Here’s our gentlesquid.
Level 18: 1 mind flayer mastermind, 1 mind flayer infiltrator, 3 grimlock followers, 2 war trolls. And here’s a pair of more traditional illithids and their thralls.
Mind flayers are such an iconic monster that even WoTC thinks it’s not D&D if they’re not in the books. And they do make excellent villains! You could have a single individual be the ultimate villain in even a mid-Heroic tier adventure by statting it up as a level 8 solo or the like.
Unlike what happens with the more naturalistic humanoids, this is one of the rare instances where I’m fine with the books portraying them as almost universally Evil. Good mind-flayers should be once-in-a-campaign events at most.