Collar

Collar (首輪) or Necklace is the name given to the spell that was used by Laura Stuart that would kill Index if her memories were not erased in one year cycles.

Principles
In order to prevent Index from rebelling, being the container of 103,000 grimoires, against Necessarius, Laura Stuart set-up the spell three years prior to the start of the series. The spell slowly deteriorates Index's body; a symptom of this is a crippling fever.

Laura Stuart had the spell put into the roof of Index's mouth, making it hidden from anyone's view. To draw suspicion away, Laura tells that Index could only contain 15% more memories as 85% of it is being used for the grimoires. She forces Index to be protected by the Anglican Church at all times, specifically, Stiyl Magnus and Kanzaki Kaori, and have them erase her memories in one year cycles lest she dies.

The Collar is apparently linked with Index's John's Pen Mode, allowing for it to be forcibly activated to defend the Collar against enemies, as well as allow it to regenerated if it is negated or destroyed, using Index's vast collection of libraries for the task. However, these safety measures were all in vain when facing the Imagine Breaker.

Index Arc
Taking the form of ♃, the for Jupiter. Touma finds the Collar, after much searching, in the roof of Index's mouth. Touma negates it with his right hand, which summons John's Pen Mode Index. The Collar is later completely destroyed by Touma after he stops John's Pen Mode, forcing it to shut down, and unable to regenerate the Collar, hereby freeing Index from her cruel fate.

Trivia

 * The collar is represented by the ♃, the.