Azoth Sword

A short sword given to Kirei by Tokiomi. It is a symbol that Kirei has completed his study of the Tohsaka Family’s magecraft. This sword is used to murder Tokiomi, and later Kirei gives it to Rin, who is unaware of this fact. As a twist of fate, ten years later in the Fifth Holy Grail War, this sword is used to deliver a fatal wound to Kirei. The weapon is not designed to be used as a cutting implement, it is to supplement and boost magecraft similar to a magic staff. The original is said to have been a sword loved by Swiss alchemist Paracelsus in the late middle ages.

Fate/Zero Animation Guide II: Glossary of the holy grail war II