There are a series of more-or-less interlocking minor quests in the Brothel of Slating Intellectual Lusts. The final one enables you to question Kesai-Serris, one of the ladies, about her mother--Ravel. She'll furnish the key you need.
But you'll also need the Unfolding Portal for the key, and that you get when you retrieve your "legacy" from Iannis, the advocate, and present it to the Godsmen's clerk, Nadilin. He'll hand you the portal.
If you want more detail, let me know, but these two locations, the Foundry and the Brothel, will certainly point you in the right direction.
Need a hint! 
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