The Devil's Blade

The Devil's Blade is a cursed object, and the weapon Cain used to murder Abel. The Devil's Blade was originally a donkey's jawbone, until it was taken to Lucifer by the devil Adirael (under Lucifer's orders) and blessed by the Fallen Arch Angel, becoming the Devil's Blade.

The Devil's Blade is the only weapon in existence capable of killing Cain for good, because God bound the Mark to the Blade. The Blade is also a very powerful object if only because not only was it blessed by Lucifer but it is also the first murder weapon. As a result, the Devil's Blade can't just be used to kill Cain, it can also be used to greatly weaken an angel or arch angel, and is also capable of exorcising a demon and casting said demon into hell just by stabbing the demon's vessel with the Blade.