The First Blade is the only known weapon that could kill Knights of Hell.
When Cain realized that his brother Abel was talking to Lucifer, and not God, Cain offered an exchange where his younger brother would go to Heaven and Cain would take his place in Hell. Lucifer agreed, on the condition that Cain himself would send his brother to Heaven. Cain then fashioned a weapon out of the jawbone of an animal and used it to kill his brother.
Lucifer gave Cain a mark, which acted as the blade's source of power. This made the First Blade powerful enough to kill anything that Lucifer could kill, including the Knights of Hell. However, without the mark, the blade would simply be a bone.
1863EditCain has since retired from his job, having met his wife Colette, who forgave him despite his deeds. However, this angered the Knights of Hell, leading Cain to destroy his very creations. In Jasper Springs, Mississipi, he hunted demons protecting the last remaining knight. To his shock, he found out that Abaddon had possessed Colette. Abaddon threatened to kill Colette if Cain did not return to his job as a knight. Cain refused and stabbed his wife with the First Blade, but not before Abaddon left Colette's body.
At this time, Cain made a promise to Colette that he would no longer work as a Knight. Since nothing could destroy the First Blade, Cain threw it into the bottom of the deepest ocean during his retirement, so that he would be able to keep his promise.
The First Blade was being located by Crowley in order to give it to its new owner, Dean, who also has been given the Mark of Cain. The two want it to kill Abaddon as it is the only weapon that can kill a Knight of Hell. Before "falling off the wagon," Crowley learned that the Blade was picked up from the bottom of the Marianas Trench by an unmanned submarine and passed among various people. Eventually Sam, Dean and Crowley track it down to rogue Man of Letters Magnus who took the Blade to add it to his vast collection of artifacts. After Magnus tries to add Dean to the collection, Dean is forced to decapitate Magnus with the Blade to save Sam, but it has a negative effect on him when he holds it. Not trusting Sam and Dean, Crowley steals the Blade until Sam and Dean can find Abaddon. Crowley later reveals where he hid it in a corpse in a grave and Sam and Dean retrieve the Blade. Dean uses it to kill a demon then face against Abaddon herself. With the power of the Mark of Cain, Dean eventually proves immune to Abaddon's powers and uses telekinesis to retrieve the Blade when he loses it. Dean stabs Abaddon in the stomach with the First Blade, killing her in a massive burst of red light. He then stabs her corpse several times with the Blade before stopping. With Abaddon dead, Sam suggests getting rid of the Blade due to the effect it has on Dean, but he refuses.
- Being the only thing that could kill the Knights of Hell, the First Blade is arguably one of the most powerful weapons in the series due to its additional effects such as telekinesis, immunity, and super strength. According to Cain, nothing can destroy the blade but, for some reason, Abaddon was confident in that she could destroy it, although it's possible she didn't know that it can't be destroyed.
- It was revealed in Stairway To Heaven that the first blade can kill anything (similar to the colt.) It is most likely that the first blade cannot kill anything that was more powerful than Lucifer, because his powers are the source for the Mark of Cain. It is most likely that the First Blade can kill anything that Lucifer could kill with his bare hands (not including God, the four Horsemen, Leviathans, and Eve.) Dean calls the First blade "A hockey stick that can kill anything" in 9x22.
- It appears that if the bearer of the Mark dies, he or she will automatically come back as a demon without being tortured in Hell.
- While when killing a normal demon, the Blade causes a glow similar to the Demon-killing knife or an angel blade, when killing a Knight of Hell it causes a massive burst of orange/red light to emerge from the Knight's vessel, similar to Cain's smiting ability, and the glow of an angel's vessel when he dies.