Mindflayers have come a long way since I first knew them.
Still they have some unexplored ecology. They are parasitic and symbiotic animals. They must eat brains. I imagine they can eat other things, but the brains perhaps feed their psionic powers better.
Its interesting that they have spell resistance – this probably manifests as a skeptical and superior view of magical things as primitive and ignorant superstition. They are compulsively evil but this is driven by the fact that they are so egoistic. They are highly introspective and poorly socialized creatures, preferring gatherings with strong social structures. They tend to be simpering, drooling when faced with evidently greater force. Their personality template is the evil Vizier in many tales of the Arabian Nights.
In combat their mind blast and other powers that they can call at will are formidable. As such their perception powers are mostly called at will. In terms of their positive personalities are very intuitive, sensitive to psychic currents of fear and anger and can learn to be remarkable diplomats when they are convinced to do it. About as easy as it is to get a computer hacker to become interested in high fashion; not a lack of intrinsic ability, but really any interest.
With their memories taken from their hosts, illithid organizations also have access to an incredible array of intelligence. They are also consumate spies if they can disguise themselves via magic, though this is distasteful, being covert is very much of their nature.
In large cities, surrounded by many many repulsive minds, their psychic perceptions would be overwhelmed and they could be turned off more often than on. This also helps keep them from being detected. A spy will tend to only use their power only when they will get something they need. their telepathy and magic resistance powers go to 100 feet. This means that they will also need to avoid defensive gaps of larger size than this nearly all the time. Larger brains have longer range and are constantly taking shifts scrying for enemy minds.
Finding an illithid spy in an ancient city would be a good potboiler thriller.
Culturally, mutual telepathy must be their primary language. I think there is a lot of polite convention and I’m sure reading another mind flayers mind requires both’s active participation but those who are read know they are being read if they are awake anyway. Reading someone elses mind is a matter of degrees and voluntarily offered information, for an illithid this must be a discipline they know quite subtley.
The mind blast is a shotgun that all illithids pack. its their weapon of choice and possibly a sign of macho power. As they have no gender, then macho has no sexual meaning. I won’t get into it too much, but they still have genetalia from their hosts and they can still breed. Not sure I want to go there, but given the general rule that crossbreeding is allowed whenever possible, the offspring may have some illithid characteristics and psionic power.
Plane shift and levitate must have a lot of consequences to the illithid life. Plane shift is a matter of local metageography. You plane shift and usually just end up somewhere. Its rare that more than one plane overlaps significantly in a given place and there are some places which have no plane shift allowed. Illithids prefer the former and avoid the latter sort of location. Planes do move slowly and so there can be seismic like shifts in such a plane shift geography, which can be of great concern to the illithids and other habitual plane shifters. The Illithid mah keep domestic forms of displacer beasts as they share this facility in common.
In military actions, it means that scouting out the neighboring planes and high ground in the other planes is a constant concern for them as they. Because they store many of their supplies and assets there, may mean that they are more vulnerable in the adjoining planes. Large encampments will have encampments in both planes and this will give them the ability to pop in and out during combat, effectively filling the space with mind flayers. They have devices to facilitate communicate between the two planes without shifting – maybe psionic communication networks from big brain vats as well.
In their every day life, the more important buildings require a plane shift to find the door. All buildings may have separate thrall entries, except illithid entries for the dead or large numbers.
Their great vulnerability are physical opponents with no minds. Machines, plants and elementals are common types of spies into their camp. There are some simple things which can block a low level psionic sweep (meant to scan for opponents). A rush of animals like a flight of birds might help. The tinfoil cap, made more ridiculous, for instance by a set of horns of about 18" in width is easily used. Simple remedies should be silly looking.
I think that all these powers must cost the illithids something, so in prolonged campaigns, their reliability must go down, they must eat.
The thrall / slave minions do much of the fighting, as well as mindflayers who have taken giants, trolls or large animals. I think these large units occur, but they consume larger amounts of brains. The tactics must resemble tank combat. Diversity in the illithid body forms must make it difficult to provision the army. The mindflayers use their own tadpoles (illithocytes) as a technology – they can create crosses with beholders, sever their heads to make intelligent vehicles, vary the tadpoles to devices of torture. any of these could have an Ulltharid variant – 10 times as powerful and as obnoxious. there really is no limitation, except the fact they all need eat brain to function. All this must mean that the thralls should outnumber the illithids by at least 10 to one in a major campaign. It would be more, but they dominate the thralls nearly to the capacity of their minds – 20-30 per platoon commander. The rest are responsible for the planar and psionic security of the camp. A minimal platoon is ideally 4 illithid and 30 thralls.
The need for food must be a big limitation on the mind flayers, they must have large thrall farms to provide food. Its not like eating a chicken – only 5% of the bulk of the food is considered desirable. The rest of the body must be like eating hamburger to them, and the brain is a symbol of status and considerable vanity amongst them. Sentient food consumes a lot of protein and so many of them must confine themselves to semi sentient food, and the ratio of illithid to thrall must be small. Looking at this an illithid army at top strength from eating brains must be pretty hard to put together -even at a brain a week per illithid. No wonder they are often found as loners. Those cities they have must have brain requirements that are pretty low. They also prefer to eat fish. An army could not go far if it ate itself; in such conditions they must keep the thralls under constant psychic dominance and there are some animals that suck will power from the thralls when the illithids are not active.
Illithid society is very hierarchical and formal, but a lot of contests of strength. small variances in their powers must constitute a huge part of that hierarchy. Its sad that they have collective tadpole raising because nepotism is one of the greatest vanities that people have. They are clearly k-type parents with probably thousands of illithocytes. So they may favor their offspring, they must only care for the spawn which survive. The big brains in vats ‘take care’ of the spawn so that only the hardy survive. This is probably a place where spawn who are less related to the brains are strongly winnowed out – a nice plot device for a specific time.