To begin this quest, go to the Brecilian Outskirts and speak to the Dalish Elves. Head into the Brecilian Forest. You'll be attacked by combinations of wolves, werewolves, bears, Darkspawn, and trees. Yes, trees. Find the Grand Oak and it'll ask (yes, ask) you to find it's acorn that was taken by a mad hermit mage. When you the hermit, you'll have to play his game. You can't attack him and take it. You have to play along. When you catch him, he'll offer you the acorn in a trade. Return it to the Grand Oak, and you'll be provided with a way to reach Witherfang. Make your way through the ruins to the werewolves.
When you reach the end of the ruins, you will be asked to return with Zathrian to negotiate. Side with Zathrian when asked and wipe out the werewolves.