This mask is the burial mask of an ancient king who worshipped Tiamat.  While wearing this mask, you can use your reaction when you drop to 0 hit points.  If you do so, roll a d20.  On a roll of 11 or higher, you regain hit points equal to half your hit point maximum, and you gain the effect of a fire shield for 1 minute (without needing concentration).  You get to choose whether it's a hot or cold shield.  The second time you use this property before a long rest, a success is on a roll of 18 or higher.  Any other times after this before a long rest automatically fail.  You also gain a +2 bonus to AC.

Notes: Bonus: Armor Class, This is based on Konahrik, the dragon priest mask from TES V: Skyrim. The original enchantment reads: When health is low, has a chance to heal wearer and damage nearby enemies (activate flame cloak)., dragonborn who is at least level 10, Healing, Damage, Buff, Headwear