The Seven Suns is a merchant organization that has its base in Southwest Baldur's Gate and operates within the Sword Coast.
In Chapter 5, the player is sent by Scar to investigate the Seven Suns. When you first enter the building a merchant inside will hail you and tell how something strange has been going on inside the organization recently. Now you can go to Scar and he will double your reward if you agree to clean out the Seven Suns. Instead, you can talk to another one of the merchants three times which will cause it and all the other merchants to turn into doppelgangers.
Or it is possible to go directly to the basement where a doppelganger and the head of the guild will be. Kill all the doppelgangers on the main floor, upstairs and in the basement and then go back to Scar and tell him about the Seven Suns doppelganger problem. If you did not talk with Scar about the shapeshifters issue, you will be scolded by Scar's lieutenant for an unauthorized action.