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An arn was a parasitic creature about the size of a small human hand, with teeth and poisonous claws, and the resemblance of a cross between a lizard and a rat, which was used in a punishment by the Rhodians against the Quill. 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 the Quill would be their slave. If they tried to remove the arn, it would kill them. (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) The telepathic link enabled the arn to distinguish Charlie from his lookalikes, however accurate they might be. For instance, Miss Quill was free to attack Charlie's doppelganger created by a bone spider from Charlie's dreams. (PROSE: The Stone House)
Dorothea Ames and the Governors, with Ballon as the surgeon, were 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)