When the Warden arrives in the Dalish Camp at the Brecilian Forest, Zathrian reveals that his clan were ambushed by werewolves and several hunters are now afflicted with the werewolf curse. Zathrian claims that Witherfang is the source of the curse -- and that he needs Witherfang's heart to try to cure his hunters before he can uphold the terms of the Grey Warden treaties.
Witherfang was a great wolf to which Zathrian bound the spirit of the forest centuries earlier in order to get revenge on the humans that murdered his son and raped his daughter. Witherfang hunted the humans, slaying some and passing the werewolf curse through his blood to others. These cursed humans were driven into the forest, and remained there when their tribe left. The curse passed on to their descendants. With Zathrian refusing every one of the werewolves' summons, they resorted to ambushing his clan to spread the curse to them and thus force a confrontation.When asked to join the cause against the Blight, Zathrian explains that the elves afflicted with the curse will soon either die or turn into werewolves, and thus they cannot uphold their obligation to aid the Grey Wardens. He states that if Witherfang is killed and its heart brought to him, it might be possible to fashion a cure for the curse. When the Warden finally arrives at the Werewolf Lair, he/she discovers that the Lady of the Forest/Witherfang desires to parley rather than fight. She asks the Warden to bring Zathrian to her in order to try to convince him to end the curse.
If you decide to side with Zathrian and kill Witherfang, it will be assisted by several werewolves including Swiftrunner. Witherfang uses many of the same abilities as wolves. Upon defeat, it drops Heart of Witherfang and an Expert Cold Iron Rune.