|Well, congrats. You discovered it. You get to name it.|
Although this article is based on canonical information, the actual name of this subject is conjectural.
This "blade" was rapier-shaped and bright gold in color. It is the only weapon to appear that was able to kill Fate as known as Atropos.
Upon Sam's questioning, Castiel remarks to him about Balthazar having a weapon that can actually kill Fate. Later, after inciting Fate, she almost kills the Winchester brothers, but Castiel intervenes. Balthazar was about to kill her with the blade from behind while Castiel was talking to her, but Castiel ordered him not to do so, because he knew that the first act of revenge from Atropos' sisters would be the death of Sam and Dean and he could not protect them anytime and anywhere.