Ceremorphosis

Ceremorphosis occurs when an illithid tadpole reaches maturity and is inserted into the brain of another humanoid being - usually a human. The tadpole eats away the victim's grey matter and essentially becomes the brain, erasing all of the subject's personality and memory, but leaving the physical body alive and under the t tadpole's control. After this "morphological transformations" occur and a new illithid is created.

Half-Illithids
If this is done with a non-human, the result is technically a "half-illithid". They do, however, still possess most of the powers of a regular illithid.