Casus Foederis

 is one of the silver swords found in the Blood and Wine expansion of The Witcher 3: Wild Hunt. While trying to enter Regis' crypt, Geralt can find an illusory wall behind which is this sword and A Knight's Oath.

Associated quest

 * La Cage au Fou (incidental)

Casus Foederis Casus Foederis Casus Foederis Casus Foederis