The bone of a righteous mortal washed in the three bloods of the fallen is a weapon that can enable lesser creatures to kill a Leviathan.
For the creation of the weapon to be successful, the following ingredients are needed:
- Bone of a Righteous Mortal - Sam and Dean chose a bone belonging to Sister Mary Constant.
- Blood of a Fallen Angel - Sam and Dean used Castiel's blood, and acquired it first.
- Blood of the Father of Fallen Beasts - The Alpha Vampire's blood was acquired second, after they saved him from Edgar.
- Blood of the Ruler of Fallen Humanity - Crowley's blood was acquired last.
After the weapon has been used on a Leviathan, its explosion will take the user and the Leviathan's essence back to Purgatory. Dean used the bone on Dick with Castiel's help and Dick exploded, he and Castiel were sent to Purgatory. Crowley said: "That bone... has a bit of a kick. God weapons often do. They should put a warning on the box."
Sam and Dean meet prophet Kevin Tran, who translates the Leviathan tablet, which says that a Bone of a Righteous Mortal dipped in the three Bloods of the Fallen is the only thing which can kill a Leviathan. Later, Castiel, being a Fallen Angel, gives them some of his blood.
Dean reads on the translation of The Word that reveals if they cut the head of the Leviathans, Dick Roman, then the body, the rest of the Leviathans, will flounder. The ghost of Bobby Singer then encourages them to contact Crowley - who is the Ruler of Fallen Humanity. He also informs them that they have to bleed the Father of Fallen Beasts - an Alpha. The boys summon Crowley who agrees to give them his blood if they acquire the other components to the weapon. He also tells them that the Alpha Vampire is still alive. Then, Dean and Sam meet with the Alpha who gives them his blood after learning that the Leviathans are planning to wipe out the vampires, along with all the other supernatural beings.
Dick attempts to convince Crowley to give the Winchesters a vial of lesser demon blood for the Bone, but Crowley gives them the true vial of his own blood instead. Meanwhile, the Winchesters seek out the bone of a nun, Sister Mary Constant, to use in the forging of the Bone. After forging the Bone, they launch an assault on Dick Roman at SucroCorp, accompanied by Meg and Castiel.
Castiel and Dean confront Dick and Dean stabs him with a fake bone. While Dick gloats, Castiel then catches him off guard and restrains him, allowing Dean to stab Dick in the neck with the real Bone. Dick, after being stabbed, starts to release a strange aura all around the room while Castiel and Dean stand confused. Dick then explodes sending black Leviathan goo all over the place. His essence is then sent back to Purgatory, but Dean and Castiel are transported there as well (it is implied by Crowley that Dean and Castiel going with Dick to Purgatory is a side effect of the Bone's use). However, the Bone has the intended effect: with Dick dead, the Leviathans are left disorganized and are effectively defeated.
- It is unknown what happened to the bone after it was used to kill Dick Roman, however, it did seem to go flying when he exploded into black goo. It was never seen or mentioned again beyond the fact that killing Dick transported Dean and Castiel to Purgatory so its unknown if Sam picked it up or the force it was blasted away with destroyed it.
- Given Dean's comment that "this thing doesn't reload", even if the bone survived the death of Dick Roman, it would not be usable again against another Leviathan and another bone would have to be made to kill another Leviathan.
- Unlike angel or demon killing weapons, The Bone does not destroy the essence of the Leviathan it is used against. Instead, the Leviathan is sent back to Purgatory. This is like when regular monsters are killed and their essences are sent to Purgatory.
- The Bone doesn't appear to need the blood on the pointed end of the weapon. When the Winchesters created it, they poured the blood on the end that was used as a handle instead.