The Demon-Killing Knife, or Ruby's Knife, is an ancient knife that was created by the Kurds which possesses the power to permanently kill most demons, hellhounds, and other demonic entities. Though it somehow came into the possession of Ruby, it eventually passes onto Sam and Dean Winchester, who use it as their most common means of killing demons.
At first glance, the knife appears to be an ordinary hunting knife with a curved, serrated-edged blade and a wooden handle. However, upon closer inspection, symbols can be seen engraved along the blade.
Powers and Abilities
This knife is capable of killing all but very powerful demons with a single stab to a vital area, such as the chest or the neck of the demons' vessels. As with The Colt, those killed by the knife briefly glow with orange light, which flickers out as the demon dies. Even non-lethal cuts are excruciatingly painful for demons in the same way that non-lethal cuts from angel blades are to angels.
The knife is also capable of harming the Horsemen War and Pestilence; it was used to cut their fingers off in the episodes they appeared in. It is also capable of killing hellhounds. However, it is not capable of killing angels - aside from the damage to his vessel, Castiel was unaffected by the knife when Dean stabbed him during their first encounter. Its effect on other supernatural creatures are unknown, as it has never been used against anything else.
But the knife has its limite powers. It can't kill the first demons like the Knights of Hell such as Cain or Abaddon. Only the First blade can destroy them.
The only subset of demon known to be immune to the demon-killing knife, are the Knights of Hell. However, the knife was capable of inflicting pain on the last surviving Knight, Abaddon, in fact it was even able to temporarily incapacitate her for a few moments. Alastair could withstand several stabs from the Knife and found contact with it extremely discomforting rather than agonizing. The angels believed the knife could kill the demon Samhain, although he also demonstrated resilience to the knife, finding cuts from it merely somewhat uncomfortable rather than agonizing like a normal demon. Even the first demon Lucifer created, Lilith, was wary of it. Cain was the only one shown to be completely immune to the knife. The Father of Murder did not flinch, wince, or show any pain at all when he stabbed himself in the heart with it. Probably, Dean, as a Demon created by the Mark of Cain, is also immune to the knife, just like Cain.
- Series creator Eric Kripke originally wanted the knife's origins to remain unknown to retain an air of mystery.  When Sam asked where she got it, Ruby deflected the question with a joke. Alastair did not recognize it, remarking that it was a "magnificent piece" and telling Ruby "you must tell me where you found it." However, the Season 8 episode As Time Goes By finally revealed that it was forged long ago by the Kurdish people, albeit with Kripke no longer as showrunner.
- Kripke refers to it as "a hand-to-hand version of the Colt". 
- A deleted scene from Frontierland shows the Colt's creator, Samuel Colt, was in possession of the knife in the nineteenth century.
- Upon Sam and Dean obtaining the knife, they have used it readily to kill demons. However, in Season 9, the brothers more commonly use angel blades, possibly because they have a wider range of things to kill (the demon knife cannot kill angels).
- In Season 10 the knife was only used to kill two demons in one episode. This is because contrary to previous seasons, demons are not a major enemy.
- When Sam attempts to use the knife to kill Crowley in The Prisoner. Crowley shows no fear calling it a tooth-pick; implying it can't kill him. However, after Dean incapacitates him with an angel blade in Form and Void, Crowley seems nervous when Dean pulls the knife, indicating that it can in fact harm him.
- Season 3
- Season 4
- Season 5
- Season 6
- Season 7
- Season 8
- Season 9
- Season 10
- Season 11
- Michael Ausiello (August 19, 2009). "Ask Ausiello: Spoilers on 'Grey's', 'House', 'NCIS', 'Bones', 'Gossip Girl', 'Supernatural', and more!". EW.com.
- Knight, Nicholas, (Season 3 Companion), p.22