Diablo 4 Season 2(Season of Blood) will begin on October 17. Season of Blood will introduce a new Questline for you to uncover, Vampiric Powers to wield against their originators, 5 additional endgame bosses with the ability to target specific Unique and Uber Unique items as a drop, and more.
Grigoire, The Galvanic Saint is a Endgame Boss.
Grigoire, The Galvanic Saint (World Tier III and IV): You must gather enough Living Steel from the Tortured Offering in Helltide. In addition to Unique items that can only drop from Grigoire, you also have a chance to receive the Demonbinder Mount Armor Cosmetic.

Echo of Varshan is a Endgame Boss.
Echo of Varshan (World Tier III and IV): Defeat Grotesque Debtors during Whispers of the Dead for a chance to receive parts of Varshan’s body. The Tree of Whispers collections—gained from turning in Grim Favors—guarantees a part of Varshan’s body. Once you have enough parts, including a Malignant Heart if you’re playing on World Tier IV, you can summon Varshan. The Flesh-Weld Rod Trophy has a chance to drop from this boss in addition to Unique items that can only drop from Varshan.

The Beast in the Ice is a Endgame Boss.
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.

Dark Master is a Endgame Boss.
Dark Master (World Tier IV): Defeat a World Boss on World Tier IV or complete a Legion Event. From these activities you’ll acquire Exquisite Blood. Once you have enough, you’ll be able to summon the Dark Master. The Diadem of the Ancient Helm Cosmetic can potentially drop from this boss, in addition to Unique items exclusive to it.

Echo of Duriel is a Endgame Boss.
Echo of Duriel (World Tier IV): Defeat both Grigoire, The Galvanic Saint and Varshan in World Tier IV to collect Mucus-Slick Eggs and Shards of Agony. Once you have enough, you’ll be able to summon the Lord of Pain, Duriel! Out of this group of Bosses, Duriel is the only one who has a chance to drop Uber Unique items. The smoldering Brimstone Mount can also potentially drop from this boss.

Keep in mind that: You’ll need to meet the summoning conditions of a boss each time you want to fight it, so hold on to any additional summoning materials that you might acquire during your travels.
5 additional endgame bosses with the ability to target specific Unique and Uber Unique items as a drop.
