After defeating Yoche, go outside and find Sihek in the southwestern part of Kurast. Talk to him and he’ll show you a house in the southeast corner of town. Talk to him there, then enter the house to find and kill an Infested Lacuni Scout.
Talk to Sihek again and he will give you the task of getting 10 insect scales from the Resentful Lacuni.
To find them, go directly southeast of Kurast, then kill them until you collect enough. Then find a diminutive camp to the west and look at the glowing objects, then return to Sihek.
He is located near the entrance to the Undercity and talking to him will unlock the Cavern District. Check the cauldron and start another run to reach the cave district.
Reach the end and kill the Lacuni boss, Longtooth, to complete the quest.
Return to the Undercity dungeon entrance and Sihek will come out. Check the tablet he left behind, then go to the council chambers in the northeastern part of Kurast to meet him and talk to Ormus.
Then talk to Sihek again to unlock Undercity’s full potential, as well as earn three “Tributes” that can be used to add modifiers (and boost rewards) to your Undercity runs. They can be found under the Dungeon Key tab on your character sheet.
With all this done, you can now face the random bosses at the end of Undercity. These runs will assist you prepare for even more arduous endgame tasks, so don’t give up!
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