Head to the Hawke Estate and read the letter on Following the Qun. After doing so visit Viscount Dumar to find that Saemus is trying to convert to Qunari. The Viscount wants you to make your way to the Docks and speak with the Arishok about this, but before you can make it you are taken to a Winding Alley. Defeat the Thugs and Mercenaries then loot them to find Helm of Enasalin / Cowl of the Overseer / Stonehammer Helm . After you speak with the Arishok head to the Chantry at night; alternatively you can inform the Viscount before going for extra conversation.
The scene that takes place is dependent on whether or not you killed Ser Varnell. If he died then you simply must fight Mother Petrice's Righteous Mob. If he lived then you can side with Petrice and fight Qunari or oppose Petrice and gain the same result as if Varnell were dead. This is the one chance you have to get The History of the Chantry: Chapter 3 so grab it on the nearby altar before the battle starts. After the battle is over a scene will occur depending on what you chose.