The Cursed Forum event is triggered when you find a Cursed Chest in a square in the Westmarch Commons in Act V. The goal of this event is to cleanse the place from its monsters (Punishers and Summoned Soldiers) after inspecting the Cursed Chest. A reward (i.e. another chest appears) if 100 monsters or more are killed during a given period of time.
Gallery
The forum with its Cursed Chest in the middle (v2.0.6)