Carnwennan

History
The two daggers, each known as Carnwennan, were relics passed down as the primary weapons of Arthur and Morgans family of assassins. They were originally wielded by Uther. Arthur took his to Camelot, and Morgan kept hers as her primary weapon. Eventually, Ten Beard took possession of Arthur's dagger long after he died.

Power
The Dagger, along with generally being strong enough to be wielded by knights such as Arthur, has the ability to turn the wielder invisible. Due to this, King Arthur was reluctant to use his after Merlin found him, only using it twice when he was otherwise unarmed and was forced to. Morgan used this invisibility to spy on and try to sabotage the Round Table, as it renders the user undetectable by anything other than high level magic, with even Merlin having some trouble breaking through its field.