Arn (species)

An arn was a creature about the size of two human thumbs, with teeth and poisonous claws, which was used in a punishment by the Rhodians. When they would release it into the brains of those to be punished, it would burrow itself inside. The creatures would telepathically link its host with a particular Rhodian, and then they would be their slave. (TV: For Tonight We Might Die)

The arn were artificially created from multiple different species. This was enough to give them a concept of belief. They believed they came from birthing grounds. (TV: The Metaphysical Engine, or What Quill Did)

At least in Miss Quill's case, if she used a weapon other than her own hands, failed to protect Charlie or attempted to remove the arn, it would kill her. She was also not allowed to tell him lies. (TV: For Tonight We Might Die)

With the help of Dorothea Ames and the Governors, Ballon was able to remove the arn from Miss Quill's head, leaving her with a scar over her eye. (TV: Detained, The Metaphysical Engine, or What Quill Did).