Ruby possessed a mysterious and presumably magical demon-killing knife, which Kripke refers to as "a hand-to-hand version of the Colt". Its handle is made of elk antlers, and the knife has engravings on both sides of the blade, although the symbols are all nonsensical.
It has been seen and used many times following its introduction in the third season. Upon being stabbed in a vital area, the demon is almost immediately killed, usually taking the human host with it. The only known survivor is Bobby Singer who stabbed himself with it while fighting off a possession and was left paralyzed as a result. It is unknown if it is effective against other supernatural beings, although it is useless against angels.
Furthermore, the demon Alastair is resistant to the knife's power as is Abaddon though it hurts both. How the knife functions has yet to be revealed, and creator Eric Kripke doubts it ever will be, stating, "I like to leave some things mysterious. And that's likely to remain mysterious." The knife's origin has been revealed however by Henry Winchester: he tells Sam and Dean that to kill a demon they need "an ancient demon-killing knife of the Kurds" and Dean tells him that's what the knife is.