It synergies well with our overall toolkit. It helps our consistent single target damage by reducing the cooldown and extending our Icy Veins with Thermal Void. Raiding - Venthyr will be our choice since Mirrors of Torment gives us essentially 3 Brain Freeze procs over 45 Seconds. On top of that, you will also be able to have multiple Frozen Orbs active. Night Fae has a lot of value in Mythic+ because it reduces the cooldown of Icy Veins with shifting power. Having Mirrors of Torment on single target works well with Icy Veins, essentially getting Brain Freeze Proc roughly every 15 seconds, which helps us extend Icy Veins with Thermal Void and reduce the cooldown of the ability due to the Icy Propulsion conduit. The 2 best covenants for a frost mage will be Night Fae and Venthyr.