Illithiderro

Illithiderro were hybrid ceremorphs that resulted from the cross-breeding of derro and mind flayers.

Ecology
However they reproduced, illithiderro were a highly prolific race of ceremorphs.

History
The illithiderro were the result of a bizarre illithid breeding experiment performed in the lower levels of the city of Gauntlgrym. In just ten years the madminds outnumbered the mind flayers and, craving freedom, rebelled and slaughtered their former masters before claiming their territory.