The quest is triggered when Hawke approaches the dead messengers camp, which is found in the eastern area of the Wounded Coast. Something seems to be making the giant spiders aggressive.
Bug!pc (1.03) If you have accepted Duty, the spot will be occupied by Flint Company mercenaries. Leaving the area and returning causes spiders to spawn, but the rest of the quest does not seem to trigger.
Hawke's party will be attacked by several waves of spiders. After continuing on, Hawke encounters a strange hunter with several mabari. Later Hawke is attacked by Sophia Dryden, a mabari and 4-5 apostatemages. The quest is completed by defeating them.