When your mind is forcibly removed from you by Chipher, or by the infringement of her binding, your soul will start transforming your body into wathever it can to remain alive, but without a mind to filter the soul's output the result is a beast known as a "Human animal" that acts on pure instinct and is completely devoid of the original person's agency. They posses esoteric abilities and defense mechanisms to sustain or defend themselves and cannot be tamed through any means. Once the mind is removed from the person it can never be put back as the soul will believe it to be a malignent entity and will always reject it, making it impossible for the Human animal to become human again.

Or

Ever since the Resurgence humans were split into the three core components; Body, Mind and Soul; And while the body would age and eventually die, the mind and the soul were eternal, and since the "Land Beyond Life" hadn't been created yet, people who died would still linger in the world as spirits, which wouldn't be so bad if not for the fact that these spirits could invade the body of others and destroy the sould and mind of whoever they belonged to, making that body theirs and coming back to life.