Elder brain

An elder brain was the final stage of the mind flayer life cycle. The elder brain lived in a brine-filled pool in the center of a mind flayer city, where it guided its community by filling them with dark dreams of illithid domination. Although its intellect was of godlike proportions, its only physical attack was by using its tendrils to lash out at or grapple an opponent. Therefore, mind flayers protected their elder brain by securing it in a well-protected cave inside the city.

Its strong mind-affecting powers stem from the brains of long-dead mind flayers making up the viscous mass of the elder brain. Being composed of many brains as energy source for its intellect it can communicate telepathically with any creature within 350 ft (107.73 m) distance.

Combat
An elder brain could use its spell-quicken abilities in the same round as its tendril attacks. Usually it started its attacks with a powerful mind blast.

In combat, it could use many psionic attacks such as charm and domination abilities. On top of those, it could cast spells as an extremely powerful sorcerer, making it an opponent to be feared.

The brain could then use its tentacles to lash out against opponents, usually those paralyzed by its psionic attacks.

Society
Forming the physical and spiritual center of a mind flayer community, it served as a living library of the community's history, technology, and knowledge. Playing this important role in a mind flayer society, it often doubled as an advisor in all sorts of political and military decisions, operating as a de facto leader.

An elder brain marked also the ultimate goal in every mind flayer's life when it neared the end of its lifespan. Mind flayers sacrificed their brain and individual life by merging with the elder brain, strengthening its powers and intellect. The average mind flayer was unaware that their personality and consciousness were lost when joining with the elder brain, and only their knowledge and ideas survived. This was a fiercely guarded secret kept by the elder brains.