After getting Kevin Tran to translate the Leviathan Tablet, the Winchesters learn of the only weapon that can defeat the Leviathans and that it requires "the three bloods of the Fallen." One of these bloods is shown to be the "Ruler of Fallen Humanity."
Trying to figure out what the bloods of the "Father of Fallen Beasts" and the "Ruler of Fallen Humanity" are, Dean gets a visit from the ghost of Bobby Singer who offers him advice on the situation. Bobby tells him that the Father of Fallen Beasts means an Alpha and that as far as he can figure, the Ruler of Fallen Humanity is Crowley, the current King of Hell. As a result of Bobby's advice, the Winchesters summon Crowley to inform him of the situation and ask him for his blood. After hearing everything, Crowley agrees to give them his blood after they get the other ingredients as his blood can be used for "all kinds of nefarious spells" against him. He does point them towards the location of the Alpha Vampire however before leaving so they can get his blood next.
Crowley is summoned into a devil's trap by Leviathan leader Dick Roman who wants to make a deal with Crowley for him to give the Winchesters the wrong blood and thus render the weapon useless. While he could easily kill Crowley, Dick knows that he probably has a vial of his blood stashed somewhere to be delivered to the Winchesters in case that happens. While they do make a deal, Crowley reneges on it as he wants Dick and the Leviathans out of the way so he can rise to power once more. When he arrives late, the Winchesters are suspicious, especially as Crowley has a vial of blood all ready for them. Crowley tells them of the deal, but insists its his blood before leaving. Later, when Dean confronts Dick Roman with the help of Castiel, Dick shows confidence that the weapon won't work as he thinks they have the wrong blood. Dean tells him that he can't trust Crowley, but it at first appears Crowley did give them the wrong blood as the weapon is ineffective when Dean stabs Dick with it. However, this is a trick so Dean can catch Dick off-guard and the blood Crowley gave them is shown to be real as Dick dies when Dean stabs him with the real weapon.
- Season 1
- Pilot (Azazel's)
- Season 4
- Season 5
- Sympathy for the Devil (Lilith's)
- Season 6
- Caged Heat (Crowley's)
- Season 7
- Survival of the Fittest (Crowley's)