Sided with the werewolves in "Nature of the Beast"
See "Poacher" for more details about this quest.
When you reach the end of the Werewolves' Lair, you'll be taken before the werewolves' leaders. Persuade them during the conversation to attack the Dalish Elves camp. You'll need to have the skill Expert Coercion (14 Cunning) to pull this off. Anything less, and they won't believe you. Once you convince them, you'll be taken the Elves camp. Wipe them out.