some old guy on the top left exit of the Karin town. He asks u to check what the weird noise is on the west side of the town and you go near the ventilation pipe until you get a quest update and go back to him. Then a chain quest appears and you'll eventually end up going in there.
Rest of the quests in regards to the dungeon should be near the entrance of the pipe or in Karin Town if you meet the correct level requirements or have done most of the quests before hand
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