Demon-Killing Knife

Ruby with her demon-killing knife in 3.01 The Magnificent Seven

In 3.01 The Magnificent Seven, Ruby appears during the battle with the Seven Deadly Sins, and kills three of the demons with a dagger. Until this point, the only way to get rid of a demon had been exorcism, which sent it back to hell, or with the Colt which killed it.

Bobby states that he has never heard of a blade that could kill demons.

In 3.09 Malleus Maleficarum, Ruby tries to use the knife to kill Tammi but loses it in the ensuing fight. Dean eventually kills Tammi with it.