During their Season 2 livestream, Blizzard revealed that Season 2 will introduce five new and returning boss encounters: Echos of Varshan, Grigoire The Galvanic Saint, The Beast in the Ice, Duriel, and Lord Zir. The development team has also shown a content map showcasing how to unlock each encounter. These bosses will be available in both the Eternal and Seasonal realms.
Blizzard also stated that these bosses are going to be permanent, that means we'll have these bosses available moving forward!
Diablo 4 Season 2 — Season of Blood Developer Update Liveblog
For Echo of Varshan, the Season 1 story boss, you will need to run Whispers, and upon completion, you will have a chance to receive fragments to summon the Echo of Varshan. When you have farmed enough of these fragments, you will enter a short dungeon to reach the end. At the end of the dungeon, there will be an interface that will allow you to summon Varshan and fight him.
For Grigoire, The Galvanic Saint, you will need to do Helltides and loot Infernal Steel from caches.
When you collect enough Infernal Steel you will be able to use that to summon Grigoire, The Galvanic Saint. This will be a lightning-based fight, and with the upcoming changes to resistances, you'll want to stack lightning resistance to make the fight easier.
Upon unlocking World Tier 4 in Season 2, Echo of Varshan and Grigoire will drop fragments that will allow you to spawn Duriel, King of Maggots. Duriel is a new Level 100 Boss that has a small chance of dropping Uber Unique items.