Holographic guardian

A holographic guardian is a security measure that Merlin was known to employ. The guardian, which took the form of a black knight, was capable of delivering physical blows to an emeny, yet projectile weapons could pass through them. They have been shown to be vulnerable to swords. Merlin had one in his vault in Avalon, to test the "worthiness" of any who would enter. He also had one guarding his library in Camelot. It is unknown if the rest of the Ancients used these.