Beginning the Quest
When you're ready to begin your third major quest out in the lands of Amaranthine, speak with Captain Garevel in the Vigil's Keep throne room. He tells you a hunter stumbled upon a mysterious darkspawn chasm in the Knotwood Hills. At the conversation's conclusion, you gain the "Last of the Legion" quest.
Amaranthine
When you enter Amaranthine, look for Colbert near the main entrance. Colbert explains that he and his partner Micah saw darkspawn emerge from a chasm in the Knotwood Hills, and he marked the spot on his hunting map. He has no interest in pursuing that sort of danger, but guesses that you do. When you're finished in Amaranthine, travel to the new location on the world map: Knotwood Hills.