Cistern in The Ragged Flagon in Riften, appears on a shelf behind the desk after completing 15 Thieves Guild jobs. If using Destroy the Thieves Guild, it will instead be located in Gray Pine Goods in Falkreath, on a cabinet behind the counter.
The Ragged Flagon in Riften
Destroy the Thieves Guild Location
Gray Pine Goods in Falkreath
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