The Demon-Killing Knife, or most commonly known as 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 antler handle (probably from some kind of deer). However, upon closer inspection, symbols can be seen engraved along the blade.
Powers and AbilitiesEdit
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 almost all 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.
The only subset of demon known to be immune to the demon-killing knife, are the Knights of Hell and Princes of Hell. White eyed demons may be immune to it as well seeing how it was unable to kill Alastair and was never tested against Lilith however it has never been confirmed that Lilith herself was immune to it. Alastair could withstand several stabs from the Knife and found contact with it extremely discomforting rather than agonizing. 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. 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 said to be 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.
- 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". 
- Gluttony was the first demon to be killed by the knife on the show.
- 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.
- 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.
- After the knife's introduction, there was a significant decrease in exorcisms. This seems fitting with the Winchesters' mentality of "shoot (or stab) first, ask questions later." In Our Little World, the Winchesters consider using an exorcism on a demon instead of the knife, but discover that the demon's vessel won't survive the exorcism due to a fatal wound and kill the demon instead.
- Sam and Dean are generally the only ones who use the knife. However, Ruby, Bobby Singer, Castiel, Meg, Becky Rosen, Gadreel, Linda Tran and Gunner Lawless have also used the knife briefly.
- In The Scorpion and The Frog, Sam calls the knife "very rare, one of a kind."
- Season 3
- Season 4
- Season 5
- Season 6
- Season 7
- Season 8
- Season 9
- Season 10
- Season 11
- Season 12
- Season 13
- ↑ 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