The loup-garou transformation is the result of a curse cast by someone very powerful, "like a major heavyweight sorcerer or a demon lord or one of the Faerie Queens."
At the rise of the full moon, the cursed person transforms into a monster and proceeds to slaughter everything it comes across until either the full moon sets or the sun rises. In their transformed state, they have supernatural speed, power, and ferocity. They are very resistant to any form of injury and recover quickly. Physical injuries sustained heal almost instantly, and they are immune to poison and to magic targeted at their mind. A loup-garou can only be hurt with a silver weapon, made from silver inherited from a family member. This fact might have to do with an element of sacrifice and is probably one of the sources for the modern myth of silver weapons and bullets being used against werewolves and other supernatural creatures.
There are several ways for the cursed person to avoid harming anyone during transformation, mostly through avoiding contact with potential victims. This can be done by leaving for a remote location in the wilderness, or by shutting themselves in securely. The cursed ones can also contain themselves in a magic circle.
According to Chaunzaggoroth, St. Patrick[Footnote 1] cursed the ancestor of Harley MacFinn to become a loup-garou. He also placed two addenda on this curse: That it would be hereditary, passed down from parent to offspring; and the MacFinn bloodline would never die out.