Placing Hevnoraak on its pedestal at Bromjunaar Sanctuary will remove any bonuses that it gained from tempering.
All dragon priest masks are considered to be circlets in terms of equipment slots, meaning that they can be worn simultaneously with some bugged items, as detailed under circlets.
As with other equipment which provides disease immunity, Hevnoraak will protect against all diseases except those contracted by traps, as detailed here.
Despite both the Resist Disease and Resist Poison enchantments being bright green in color (when viewed from the inventory, on other pieces), the enchantment color of Hevnoraak is red.