Arondight

This is the sword prized by the knight who was once admired as the best. As it has the same origins as Excalibur, a weapon created by gods, its toughness can match that of Excalibur.

It is a sword that is allowed to be carried only by the best knight of the current era, but as the knight slew his brethren, it lost its status as a holy sword and gained the attributes of a demonic sword.

Source: Fate/Zero vol.3 status page and Fate/Zero vol.4 weapon page