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Collar is the tentative name of a magic item owned by Ainz Ooal Gown that can enhances one's experience and level up.
The Lizardmen were given these items by Cocytus to allow them to level up at a higher rate. Ainz used one himself during his battle with Foresight.
A leather collar, with a chain, broken halfway down, dangling from it.
An item that can allow the wearer to earn a higher rate of experience point gains for leveling up. However, the setback when it comes to using this item was that it leaves the wearer greatly weakened during the process of acquiring XP.
The item's effects and purpose are implied in Volume 7 and Volume 10.
Aside from Ainz and the lizardmen, the Death Knight was also said to have been equipped by a similar item like the Collar or possibly the same one.