Illithids outside of Genesis

Illithids are evil, sadistic beings that eat brains. They have numerous face tentacles, and a lamprey like mouth, which they use to feed on brains. In common mythology, they have an Elder Brain, which acts as a leader, and lives in a pool of cerebral fluid. Each illithid spawns twice in their life, which are creatures resembling an illithid's head. It will attach itself to a sentient humanoid (human, elf, orc, goblin etc) head.

Illithids need the brains of creatures to live. A typical, evil illithid will seek for brains of sentient beings, and consume it while the victim is alive. Animal's brains are fine too, as well as brains from unliving creature (but where's the fun in that?). They are amphibious creatures, who require water to live. They loath the sun, and have eyes that can see perfectly well in light and darkness.

Illithids generally consider themselves superior to others, whom they consider as food and slaves. Most who face the fearsome tentacled beings will have their minds flayed by the mind flayer's psionic shockwave. Undeads and skeletons are not affected. They are also capable of enslaving beings with their minds, as well as summon a psionic force field.

Illithids in Genesis

Illithids in the Genesis mod are powerful fighters compared to the serpentfolk and demonic union. They use weak equipment made of copper and bronze, but don't let that fool you, for they do not require powerful armor, as their force field is as powerful as steel. Some variants have boiling blood and breath attacks. They also spawn tadpole like creatures like 'real' illithids, although Dwarf Fortress mechanics prevent the latch-onto-sentient-being's-face. They also do not feed on brains like their 'real' counterparts, but will readily abuse bodies.

They are ambushers as well as siegers, and are capable of swimming (allowing them through the river, and murky pools and break down the irrigation floodgates with building destroyer mounts, then swim out relatively unscathed).

Illithids are also thieves and baby snatchers and live in dark towers much like vanilla goblins. There is a variant of illithids that live in caverns like the animal men tribes.

Fighting the menace

Use trained soldiers and traps. Do not forget some illithids can fly, or that others may ride giant bats and other flying creatures. Do not underestimate their equipment for weakness, because they have (as mentioned above) force fields as thick and strong as steel. Traps also help. Despite all their powers, they are no match for fully trained and equipped (as in good old steel, or a certain blue rockā€¦. and i don't mean microcline) dwarves. Marksdwarves with steel bolts should also take them down with ease.

All that can be summarized with this : they are vanilla goblins with steel and a few little tricks up their force field covered sleeves.