If an illithid hero attempts to use their psionic abilities on a beholder, it fails and they get confused instead, because the beholder’s mind is so alien that they can’t comprehend it.

Your chance of sucking an opponent’s brain increases the hungrier you get.

When polymorphed into a mind flayer (or playing an illithid in variants that have it as a race), you gain additional nutrition when you eat the brain of a spellcasting or telepathic monster.

Illithids and illithid-race players can only eat brains and corpses that have brains. If you kill a monster after eating its brain, its corpse offers no nutrition.