This quest is gained by talking to Aldeth Sashenstar in front of the Merchants' League Estate. In order for him to be there, Gorion's Ward and their party must have sided with him at his hut in the first area of the Cloakwood; otherwise, his brother, Dabron Sashenstar, will be there and will attack you. When you talk to him, he'll say that something is off at the Merchants' League and that his two partners, Irlentree and Zorl, have been acting weird and making bad buisness decisions. You'll be asked to investigate this and invited inside the building. Once inside, go to the second floor; in a locked desk in the southern room, there are documents from which you will learn the truth: Irlentree and Zorl are actually doppelgangers. From here you can choose one of two paths:
  • Talk to and give the documents to Brandilar - a guard on the second floor. He wants you to kill the doppelgangers right away and, immediately after the conversation, the merchants will revert to their true forms and become hostile
  • Give the documents to Aldeth, and he will lead you the third floor to his fake anniversary party, where you will confirm what was in the letters and learn that the doppelgangers were sent by their master, Rieltar. After the dialogue, all the merchants in the room will revert to their true forms and turn hostile

No matter the path you chose, after you have killed all the doppelgangers, talk to Aldeth to get your reward.