An intellect devourer is a small slug-like creature used by Illithids to control the minds of sentient beings. They seek out intelligent life and then enter the victim's skull through any opening it can find - eyes and ears being common choices. Once inside, the creature latches onto the brain of its victim and assumes control, working much like a one-way remote for whatever being is in control of the intellect devourer.
This process is thought to be irreversible, as removing or destroying the intellect devourer invariably kills the host as well.