Blue Dragon Mask
Wondrous Item, legendary (requires attunement)
This mask of glossy azure has spikes around its edges and a ridged horn in its center. The mask reshapes to fit a wearer attuned to it. While you are wearing the mask and attuned to it, you can access the following properties.
Damage Absorption
You have resistance against lightning damage. If you already have resistance to lightning damage from another source, you instead have immunity to lightning damage. If you already have immunity to lightning damage from another source, whenever you are subjected to lightning damage, you take none of that damage and regain a number of hit points equal to half the damage dealt of that type.
Draconic Majesty
While you are wearing no armor, you can add your Charisma bonus to your Armor Class.
Dragon Breath
If you have a breath weapon that requires rest to recharge, it gains a recharge of 6.
Dragon Sight
You gain darkvision out to 60 feet, or to an additional 60 feet if you already have that sense. Once per day, you can gain blindsight out to 30 feet for 5 minutes.
Dragon Tongue
You can speak and understand Draconic. You also have advantage on any Charisma check you make against Blue Dragons.
Legendary Resistance
(1/Day) If you fail a saving throw, you can choose to succeed instead.
Lingering Shock
If you deal lightning damage to a creature, it can’t take reactions until its next turn.
Source: The Rise of Tiamat Online Supplement p. 4