The loup-garou transformation is the result of a curse; the person making the curse has to be very powerful, "like a major heavyweight sorcerer or a demon lord or one of the Faerie Queens." Harley MacFinn's ancestor, for example, was supposedly cursed by Saint Patrick.
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.
A part of the curse is that it prevents the family line from ever completely dying out.
The Beast of Gévaudan that was active in 18th century France was a loup-garou; it was also killed with a silver weapon.
In Fool Moon, there is another loup-garou.