From the histories of Clan Torvin, presumed lost in the Mage-tears Range ten years ago:
The first Warg was found by Clan Morgris, starved and near to littering. They took it in, found that they could bond the pups to their own, and that the change bred true with the dog packs they had. Much of the Morgris wanted to keep this new advantage for themselves, which led to the Clan-Wars [history kept by Clan Kaevorn]. In the end all the Clans received a Warg-bond. But while it provides an advantage in battle, those who have bonded take on the characteristics of the Warg, and the Wargs take on the traits of their partners, with the end result being a complete change of form for both, and by this means the Clans and Packs are strengthened. But until the change is complete they pay for their choice; they become outcasts, forming their own groups until the self-shifting is complete.
Later note: Created for a Reddit challenge in /r/rpg.