Illithids, or Mind Flayers, are alien or aberrations, so they can live a long time. So these long lived highly intelligent and psionic creatures are running around, they are going to have lots of minions.
- They could be a creator race, they create Behir, Bugbears, Carrion Crawlers, Cave Fishers, Chimera, Displacer Beasts, Ettins, Lamia, Remoraz, Stirge and Umberhulk as new servants.
- After the Githyanki rebelled, the Mind Flayers needed more allies, and created Ankheg, Doppleganger, Gargoyles, Grell, Grick, Hobgoblins, Intellect Devourers, Ogres, Owlbears, Piercers, Purple Worms & Troglodytes.
What this means is these creatures will not attack Mind Flayers, they are magically or genetically coded to be passive towards them.
Thus before you encounter any Mind Flayers, you are going to be dealing with many of its minions first, they are highly intelligent and don’t want to get into combat until the prey is weakened and they can dine without getting a sword in the guts.
Since Drow society is self-destructive in that it competes against itself and the Duergar are slow breeding, I see Mind Flayers as the major players for the Underdark etc.
Intellect Devourers could have been an attempt of Mind Flayers to create better prey out of cats, dogs and rats. In the end they just treat them as guard dogs. One option is some Intellect Devourers may aid the PCs in return for freedom or a chance to increase their quality of life.
You don’t need a community of 1000 Mind Flayers, two hundred would be a large threat for the continent or 3-5 nation area. I’d go for 200 hundred Illithid, then 2000 minions/thralls etc.
Now the Astral War (Mind Flayers vs Githyanki vs Githzerai) can be a major factor in your campaign, with each side having a variety of allies or minions with the ongoing goals of keeping an eye on the other two sides, and taking them out whenever possible. When you talk about the history of Dwarves and Elves you are talking about being around for thousands of years. When you talk about Mind Flayers and Githyanki you are talking about millions. So they have been gathering forces, fighting and interacting with multiple worlds (even alternate worlds?) for a very long time.
With all this in mind, to many species they would see Mind Flayers as an elder ancient race, behind many of their enemies. For example Halflings could have a bunch of legends about Hobgoblins and Bugbears, but in some of those legends the Mind Flayers are directing the Hobgoblins and Bugbears.
If you are thinking hey these guys are just too powerful, don’t panic. There are a range of creatures that could notice their activities (thanks to long life, magic and intelligence) to aid the good guys. Eg-Demigods, Dragons, Giants, Phoenix, Planeswalkers, Sphinx, even Vampires (who don’t want competition for the food)
Using Mind Flayers in games – Don’t think small, they should be a major threat and intelligent.
Idea 1 : In a possible future or alternative present – the Mind Flayers won, the sun is gone or darkened greatly, there are slave camps everywhere, and whatever pockets of the resistances are the players. Maybe the characters in the regular game need a person/item from the ‘other world version’ since in this world the person/item never existed or was stolen/damaged.
Idea 2 : Mind Flayers want to block out the sun, so they want to gather Pelor/Sun God worshippers, and use them (alive) as part of a ritual, this ritual must be performed during an eclipse or requires 100+ worshippers etc. As a twist/complication, in one area the mind flayers could use their agents to sponsor poor people into joining the Pelor cult, just to get more prey later.
Idea 3 : The Illithid (and their minions) are aiding the less intelligent species such as Goblins, Bugbears, Lizardfolk, Grimlocks, Orcs etc. teaching them skills, tactics, magic, languages, all sorts, and have been for some time. This creates a better food menu for them, and gives them a better class of minions against their enemies. The problems of more intelligent Humanoids running around is an escalation in well executed attacks against PC races. (This may have happened in the past and led to a change in era/epoch etc). As a further twist, they are also sponsoring Psionic or Arcane guilds in PC races to promote learning-increasing intellect, why? Again to provide better food/minion choices, and the arms race-conflict itself between more intelligent foes will also escalate the evolution of their prey.
Idea 4 : Mind Flayers and their minions journey to the surface during each of the sun(s) eclipse with the moon(s). During this 3 to 6 hour period (your choice) they can come out and have a sacred hunting time, which of course is a nightmare for the surface races. For that matter for the Duergar & Drow communities they go past or near on the way, it’s not nice either, so they may even aid the PC races with information (enemy of my enemy etc).