The Beast in the Ice (World Tier IV): Gather Distilled Fear while completing Tier 30 or higher Nightmare Dungeons. After you have enough, visit the Occultist and craft the Glacial Fissure Nightmare Sigil, activate it, then you can fight The Beast in the Ice. The Skull Torch and Fell Steed Mount Trophies can potentially drop from this boss, in addition to Unique items exclusive to it.
All this is nice and all, but... it's boring to spam nightmare dungeons as the only leveling path possible through all the way to level 100 every season.That's one of the big factors in player drops. Blizzard created a "mmo lost ark clone" in diablo 4 with a big open world full of events, side-quests, etc. Use it, make the damn seasons memorable.
I suppose that the Sigil for the boss is not permanent but it will be one kill per sigil?