Bone of Righteous Mortal Washed in the Three Bloods of Fallen

A bone of a righteous mortal washed in the three bloods of the fallen is the only weapon that can enable lesser creatures to kill a Leviathan.

Creation
For the creation of the weapon to be successful, the following ingredients are needed:
 * Bone of a Righteous Mortal: The boys choose a bone of Sister Mary Constant.
 * Blood of a Fallen Angel: The boys 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 the Leviathan.
 * Blood of the Ruler of Fallen Humanity: Crowley's blood was acquired last.

Side Effect
After the weapon been used in a Leviathan, its explosion will take the user 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."

Reading Is Fundamental
Sam and Dean meet prophet Kevin Tran, who translates The Word of God, 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.

There Will Be 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.

Survival of the Fittest
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.