The Dragonborn learns from Wujeeta, a Skooma addict working at the Riften Fishery, that Sarthis is producing and dealing Skooma out of the warehouse. After informing Jarl Laila Law-Giver or Maven Black-Briar of this, she sends the Dragonborn to kill Sarthis and gather information on his operation.
Upon entering the warehouse, if detected, both Sarthis and Orini will attack the Dragonborn. Once they are dead, a search in the basement will uncover Sarthis' satchel, which contains a small amount of gold and a note titled Shipment's Ready. The note identifies both where and from whom Sarthis is acquiring the Skooma.
Once the note has been delivered to either Jarl Laila or Maven, she will send the Dragonborn to Cragslane Cavern to eliminate the supplier and put an end to the operation.