Mind Flayers or Illithids from Dungeons and Dragons are some of the most unique enemies in the fantasy medium. They barely qualify as fantasy creatures and seem to tiptoe the line between fantasy and sci-fi. At their core, their Lovecraftian origins are based on their tentacles and the fact that they're telepathic creatures with psionic abilities.
Additionally, their appearance is about as enigmatic as their background. They're also troublesome as adventure enemies. Most players who encounter them don't bother learning or even remembering much about these grotesque and perverse squid creatures. That's quite a shame because they have some of the most interesting and argued lore in the Forgotten Realms.
10 They're Not From The Forgotten Realms
Well, scratch the "Forgotten Realms" part, as the Mind Flayers or Illithid likely didn't even originate in that realm or universe. They are classified as Aberrations which are otherworldly enemy types (they usually contain tentacles or incomprehensible inhuman physical traits) and most of those come from the Far Realms.
Most creatures from the Far Realms are described as maddening when gazed upon by the simple and feeble minds of other beings. It's basically the Dark Web of the D&D multiverse lore. The Illithid don't just reside in the Forgotten Realms but other realms apart from the Far Realm as well. In the Forgotten Realms, they holed up in the Underdark due to their natural aversion to sunlight.
9 They Smell Like Cooking Ingredients
The next time a DM describes a Mind Flayer, make sure they up their Matt Mercer effect by telling them beforehand that Mind Flayers have their own distinct scent. Apparently, the healthy ones who snacked too much on adventurers' brains excrete an excess of mucuson their skin to maintain moisture.
This mucus thus exudes a fragrance that either smells like garlic, onions, or vanilla. The odor isn't that strong, of course, but in the Underdark, that should be a good indication of a Mind Flayer presence. Some Mind Flayers don't have this smell though, particularly the unhealthy ones.
8 Most Of Them Are Physically Weak
One of the reasons why Mind Flayers keep floating around is because their legs and limbs are weaker than a typical humanoid in the Forgotten Realms. It could be an evolutionary handicap as they might have relied too much on levitation to the point that they weakened their limbs. Nevertheless, they're pretty terrible as melee combatants.
Hence, most Mind Flayers rely on psionic or energy attacks that target their victims' minds. Since their staple diet is brains, then it's safe to assume that their psionic attacks are best treated as meal preparations – tenderizing the meat, so to speak.
7 Undead Mind Flayers Exist
With that said, some Mind Flayers are aware of their physical limitations and became rebellious and ambitious enough to learn magic (separate from their psionics). They then followed the path of the evilest magic users in D&D lore: the road to Necromancy.
Thus, undead Mind Flayers or Alhoons, a.k.a. "illithiliches," do exist in the Forgotten Realms. Much like other undead in the Forgotten Realms, they're shunned by their own living kind and thus band together in closely-knit groups as few as they are, in order to deal with their enemies.
6 They're A Hive Mind
Most Mind Flayers, however, don't become undead or too ambitious, as they all answer to the Elder Brain. This Elder Brain telepathically controls them and gives them orders and tasks. Thus, it can also be said that there are times when Mind Flayers don't act of their own will.
Even if they do, they still work hard to avoid the ire of the Elder Brain. Any Mind Flayer who becomes too intelligent, ambitious, or rebellious can actually challenge their Elder Brain or get kicked out of the group so they can establish their own colony as their own Elder Brains. Alhoons are outliers among these.
5 They're Sworn Enemies Of The Githyanki
Despite their reputation as the feared overlords of the Underdark, some brave races in D&D still managed to muster up their forces and courage to stand against the Illithid. The most prominent enemies of the Mind Flayers would be the Githyanki, or Gith.
Initially, they're actually a race of humanoids whom the Illithids enslaved among many. With the help of dragons, the Gith managed to liberate themselves and have sworn to dedicate their existence to waging war against the Mind Flayers whenever they can.
4 Even Aboleths Fear Them
Another race of creatures that detest the Illithid is the Aboleths. These are whale-like Aberrations that are also one of the oldest creatures to exist in the Forgotten Realms. They hold themselves in high esteem and are said to have memories of all races in the Forgotten Realms – except for Illithids.
That's probably one of the reasons why they fear the Illithid, which is odd for them since even Aboleths aren't afraid of gods (they witness the gods' ascension and birth, after all). They simply do not recall the creation of Mind Flayers, so to them, the whole race is alien and unknown, hence they have come to fear the Illithid.
3 Their Goal Is To Restore Order To The Multiverse
So what do the Illithid or Mind Flayers want? Well, all their Elder Brains have one common goal. If they're not planning to blot out the sun that annoys them, they're plotting to "restore order" to the multiverse. They believe that they're the most superior race of all and that all other inferior races are causing too much chaos and destruction.
Hence, in their mind, it falls upon the Illithid to clean up whichever realm they land upon. That also explains why they tend to assimilate humanoids and other races in their ranks and convert them to Illithid. That's how they expand their colonies.
2 They Practically Have Spaceships
Of course, the Illithid wouldn't have considered themselves superior if they don't have the technology to back that up. Reinforcing the idea that they're alien is the fact that they have what's essentially spaceships that allow them to travel dimensions and realms.
These can fly, kidnap and store victims to assimilate, and even fight. It's called the Nautiloid, though they don't exactly have enough firepower to face dragons and win. Nautiloids also allow Mind Flayers to travel on the surface while shielding themselves from harmful sunlight.
1 Mind Flayers Are Possibly From The Future
If it wasn't obvious already based on the Aboleth's fears and their spaceships as well as their superiority complex, the wildest theory about the Illithid is that they're actually from the far future. The assumption is that they're an advanced civilization near the end of times who witnessed their kind fall into demise.
That somewhat explains why they went back to the past (roughly thousands of years before any adventure in the Forgotten Realms) to change their apocalyptic outcome in the future. That did mean they reduced themselves to a shadow of their former empire, but if that theory holds merit, then the future of the Forgotten Realms is looking rather fishy.