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.