The Beast in Ice boss is definitely one of the most elaborate to summon in Diablo 4's Season of Blood. While it isn't quite as complex as Duriel—where you have to fight both Varshan and Grigoire to gather the necessary materials to summon him—this icy demon is tied to the Nightmare Dungeon activity, which brings problems of its own.
There isn't really much of a bottleneck in terms of getting materials to fight Grigoire and Varshan, but since The Beast in the Ice requires Nightmare Dungeon completions for even a chance of getting the Distilled Fear you need, it takes a hell of a long time. So long, in fact, that I'd recommend waiting to see if Blizzard tweaks the drop rate. Since the boss level is 85, there really isn't too much rush to fight him.
Still, if you're intent on besting the beast, here's what you need to summon The Beast in the Ice boss. Be sure to see our Diablo 4 boss loot tables if you want to know what Unique items he has a chance of dropping.
Where to find Distilled Fear
You can get Distilled Fear by completing tier 30 and above Nightmare Dungeons on Torment world tier. Since the material drops randomly on completion, you might not always get a vial—besides Duriel, The Beast in the Ice has the grindiest boss material farm. It took me 11 Nightmare Dungeon completions to get the necessary components, and that's apparently a favourable drop rate.
You need nine Distilled Fear overall in order to summon The Beast in the Ice, so it's recommended you try and complete the shortest Nightmare Dungeons you can, like Guulrahn Canals or Blind Burrows. Once you've finally farmed up the Distilled Fear necessary, you're ready to summon the boss.
How to summon The Beast in the Ice boss
Before you can access The Beast in the Ice boss lair, you need to use that Distilled Fear to craft a special Nightmare Dungeon Sigil at the Occultist. If you open the Ancestral section, this is the first option available to craft: Glacial Fissure Nightmare Sigil.
"Why does this sigil have affixes?" you may ask. Well, the bad news is that fighting The Beast in the Ice is treated like a Nightmare Dungeon, meaning you only have three revives overall to beat it. I don't recommend attempting The Beast in the Ice until you're at least level 85. Considering the grind even a few attempts require, it's not worth trying until you have a decent chance of success.
Once you arrive at the Glacial Fissure lair dungeon near Kyovoshad, you'll have to fight through hordes of enemies to get to the boss arena entrance. Unlike the other bosses, The Beast in the Ice will drop into the middle of the arena straight away. In general, look out for his ice breath attack which creates little projectiles, and when he jumps into the air to try and squash you.
If you manage to reduce his health, he'll summon a storm which traps you in the centre of the arena with some enemies—look out for the strafing AoEs he also summons here. Make sure to slot Sapphires onto your jewellery for that extra cold resistance.