This is one of eight stone masks, created and worn by those on the Council of Mages in the Findalan Magocracy. While attuned to this mask, the wearer is immune to the Scrying spell and other Divination magic. In addition, the wearer gains a benefit determined by the style of the mask:

Abjuration (Turtle) - You know the Blade Ward cantrip. When you reach 3rd level, you can cast the Shield spell once per day; you must finish a long rest in order to cast the spell again using this trait. When you reach 5th level, you can also cast the Lesser Restoration spell once per day; you must finish a long rest in order to cast the spell again using this trait. Intelligence is your spellcasting ability for these spells. When you cast these spells in this way, they do not require components.