This mask is the burial mask of an ancient king who worshipped Tiamat.  When worn by a dragonborn, it gives them expertise in thieves' tools and alchemist's supplies.  In addition, any ranged attack you make while attuned to the mask has a +3 bonus to attack and damage rolls.  You also gain a +1 bonus to AC.

Notes: Expertise: Alchemist's Supplies, Expertise: Thieves' Tools, Bonus: Ranged Attacks, Bonus: Armor Class, This is based on Krosis, the dragon priest mask from TES V: Skyrim. The original enchantment reads: +20% to Lockpicking, Archery, Alchemy., dragonborn, Damage, Buff, Utility, Combat, Headwear