Anti-Magic Mask

Anti-Magic Mask (抗魔の仮面) refers to a mask first seen to be owned by Hero Chronoa. It was given to Shizu to suppress Ifrit, who resided inside her. It was later passed down to Rimuru Tempest upon Shizue's death. Rimuru then gave it to Chloe Aubert as a parting gift after his time as her teacher was over.

While the real mask was in the hands of Chloe, Rimuru was still able to duplicate a working copy of the mask, though Rimuru modified it to only have the Magic Resistance ability in strengthened form, as the other abilities were useless to him.

During the events of Volume 11, it was revealed that Chronoa and Chloe are one and the same, and she possessed the mask in the past because Chloe brought it back to the past during her time travel. This means the mask is paradoxically cycled between its users (Chronoa/Chloe -> Shizu -> Rimuru -> Chronoa/Chloe) during these countless time loops, with its true origin unknown.

Ability
The mask is a magic item infused with four effects:
 * Magic Resist: Dulls any magic attacks that the enemies cast and hide the user's internal magical force.
 * Antidote: Nullifies poison that made contact with the user.
 * Breathing Support: Allows the user to keep their lungs full even if the flames consumed the oxygen around the user.
 * Amplify Senses: Increases the user's senses.

Since the mask is the crucial object that cycled countless times in Chronoa/Chloe's time loop, the power of "time" element accumulated on it is immeasurable. It's so much that it could deflect a direct hit from the Black Primordial, Noir, and had his arm blown off by the recoil.

Noir became obsessed with the mask and had been closely following Shizu's activities ever since their encounter. The reason that Noir is fascinated with Rimuru, is because he witnessed the mask crack in Rimuru's presence, suggesting he is the key person related to that mask.