Ok so there are 3 new masks that have been added to the game from the dragon born dlc recently and thos 3 are:
Zahkriisos
Dukaan
Ahzidal

These are all specfic to battle mage type characters and provide boosts in each destructive magic genre.

Zahkrissos is for those who prefer to fight with the electric destruction magic
Azhidal is for those who like to fight with fire destruction magic
Dukan is for anybody who uses frost type magics.

All of these masks provide a 50% resistance to shock, frost and fire damage they also increase damage by the spells by another 25%
so say you use flames eg a fire type spell this will increase the damage out put by 3 equalling 15 damage over time on an enemy but you will need to wear the azhidal mask to be able to do this.

There is also miraaks mask which i was rather disapointed by as there is no special effect as good as the armour itself it only increases you magicka by 70 points eg 300+70=370 magicka
Miraak is the boss specfic mask used by the antagonist himself.
I hope this clears up any questions about the new dragon priests with the dlc and i will hope to add more if i find anymore masks or priests in the game