The Blade of Oberon was a magical weapon which was faerie property , a faerie dagger and a national treasure It was capable of killing ghosts by draining their essence and scattering their memories, personality and talents. It was first seen by humans in the fifteenth century, after the Elf King of Bordeaux duelled with the Blackmere Spectre. It was used to reverse the usual process of dispersion of the spirit's energy to instead render the spirit into a super-strength ectoplasm.