Silver is an metal that all Werelords fear. It is the only substance that can't be healed by therianthropy. Captain Brutus had it on his Lionshead blade and used it to kill Baron Huth. Also, Mack Ferran's Wolfshead was treated by silver after his son Trent possessed it.
Forbidden in the reigns of Wergar and Leopold, thou the latter did grant the use of the material to select members of the Lionsguard.