1. Collect Shards of Agony and Mucus Slicked Eggs
In order to summon Duriel, you’ll need: 2 shards of agony and 2 mucus slicked eggs.
How to Get Shards of Agony
These are obtained by defeating Grigoire, the Galvanic Saint located in World Tier 4.
To summon Grigoire, you need 5 Living Steel, which can be collected from Tortured Gifts of Living Steel during Helltide events or other activities like Tree of Whispers.
Grigoire is a challenging boss that deals significant lightning damage, so equip Lightning Resistance gear and use Lightning Resistance Elixirs.
Each time you defeat Grigoire, there is a chance to receive a Shard of Agony, so you may need to summon and defeat him twice to receive the 2 Shards of Agony required to summon Duriel.
Pro tip: Only the player who summons Grigoire will receive a Shard of Agony upon defeating him.
How to Get Mucus-Slicked Eggs
Mucus Slicked Eggs are acquired by defeating the Echo of Varshan, but before you can summon him, you need to acquire Malignant Hearts.
Malignant Hearts can be obtained by completing Grim Favors from the Tree of Whispers on World Tier Four or by defeating Grotesque Debtors found in Legion Events or near World Bosses.
Once you have them, travel to the Writhing Mire dungeon located in the northeastern area of Hawezar (right by the Tree of Whispers). Inside this dungeon, locate the altar to summon Varshan. Defeat Varshan to acquire Mucus-Slicked Eggs.
2. Summon Duriel for Battle
Now that you have obtained the necessary materials, it’s time to place them at the altar and summon Duriel for battle.