This quest is only available after you have completed the quest Long Way Home.
Talk to Solivitus in the Gallows, who will ask you to gather three herbal resources for him. This quest is offered in each act, with different resources to find each time.
You need to acquire the following resources for Solitivus:
You must first talk to Master Ilen in the Dalish camp on Sundermount. He will mark a new area, the Ironwood Copse, on your map. After killing all the monsters in the area, mostly Hurlocks and Hurlock Bolters, the Ironwood will be marked with a plot point and can be collected.