Alhoon
Alhoons are undead illithids in the realms of Dungeons & Dragons. Sometimes presented with the alternative name of Illithiliches, which other sources claim are an entirely different thing, alhoons are mind flayers outcast from their society due to their study of the forbidden arts of magic over psionics. As this denies them their culturally-promised immortality through joining the Elder Brain upon death, alhoons instead seek their own immortality through undeath, becoming lich-like monsters that often prey on their living kin.
5th edition makes alhoons and illithiliches separate things, with the former being the result of illithid spellcasters taking a shortcut, and attaining a lesser form of lichdom that's easier to achieve, and the latter being the real deal holyfield, losing none of their mind flayer abilities (alhoons can't eat brains anymore) and gaining only advantage from their undead state.