To summon bosses, you will need specific materials such as Exquisite Blood in Diablo 4, which is for Lord Zir. He is an Uber Boss in the Ancient's Seat Dungeon in the Darkened Way in Fractured Peaks. When you spawn at the boss location, turn in Exquisite Blood at the Bloodied Altar and watch as he appears, preparing to be farmed by your character.

To obtain and farm Exquisite Blood, you will need to play Helltide events, specifically the Blood Maiden that spawns at the Accursed Ritual. This can be triggered after contributing 3 Baneful Hearts. Note that the bosses can be relatively tough unless your build is overpowered, but fellow players can help you lower their health until then. Farming this event will give you a lot of Exquisite Blood at D4 and is the most efficient way to obtain this resource.

Additionally, you can also complete Legion and World Boss events whenever they appear on the map. There’s more — the Tree of Whispers caches can also drop this resource. Speaking of caches, you can also have a chance to obtain Exquisite Blood from the Season Journey reward caches.

Finally, if you see a Treasure Goblin trying to escape, make sure to take him out and collect the loot. It may also contain Exquisite Blood.