You will get here after the task "The Embassy". These lands are beyond the Barren Light. This location is more or less a huge location with different places. So, there will be more quests here.
The quest will start automatically after you complete the "Shadow from the Past" side mission in Barren Light.
While traveling through the Desolate Lands, communicate with different characters (marked with green markers), and from one of them you will receive this task.