The Risk Dagger is a blade once own by the Witch-King of Angmar. After the Witch-King's injuries during the War of the Ring, he went to live with Darth Vader. During this time, when Vader and the Witch-King were squabbling, Radagast and the Doctor appeared in the TARDIS and stole the blade.
It soon fell into the possession of none other than Mr. Risk. He would use it to counter Risks in the world, especially when children were under attack by Le Bougne. He lent the blade to one of the kids who wounded Le Bougne, driving him away.