I have a (slightly) different take on illithid: The elder brain makes tadpoles. Tadpoles will eat their way into anything with a brain, and do their level best. May become a mind flayer, may become some other horrid beast. Mind flayer lairs are choked full of these fun experiments.

At the end of their life cycle, all brains are returned to the elder pool, where they are rendered down - they feed the elder brain, not just in a nutritional sense, but it also absorbs their memories.

Tossing dead bodies into the pool also allows the little tadpoles to gnaw their way in, creating either a mind flayer (if the body died, like, last round) or something I call a mind thrall - basically a psionic zombie.

And of course the illithid can use alchemy to brew all sorts of potions from the brain juices of the various creatures they've enslaved.