This skill, Avalon, is property of Ishimura Elite.
Avalon allows Knight to defend against all attacks without recieving a single point of damage. This can be used on himself or others to defend them in emergencies. This defense is completely indestructible by any means, and no weapon in SAO can penetrate the defense while it is active (with the exception of Skeith, and possibly Judgement Cut, although that has not been tested). An extremely long cooldown time ensures that Knight can only use it as a last ditch defense to protect himself or others.
This skill is linked to the sword Excalibur, and therefore can be used by anyone wielding the sword.