The Flame of Time
When an illithid undergoes ceremorphosis, it can occasionally take on some elements of the absorbed host creature’s former mind, such as mannerisms. This typically manifests as a nervous habit and/or reaction, like nail-biting or tapping one’s foot. An adult illithid has even been known to hum a tune that its host knew in life. Usually, when a mind flayer inherits a trait like this, it keeps it a closely guarded secret, because, were its peers to learn of it, the illithid in question would surely be killed. This is due to an illithid legend of a being called the Adversary. The legend holds that, eventually, an illithid larva that undergoes ceremorphosis will take on the host’s personality and memory in its entirety. This “Adversary” would, mind and soul, still be the host, but with all the inherent abilities of an illithid.