|“||Always happy to bleed for the Winchesters.||”|
— Castiel while giving the brothers his blood
After Kevin Tran translates the Leviathan Tablet, the Winchesters learn that the first ingredient they need for a weapon to defeat the Leviathans is the blood of a Fallen Angel. Now considered a Fallen Angel, Castiel happily donates his blood for the weapon, telling them "you know me: always happy to bleed for the Winchesters."
While trying to figure out the rest of the weapon, the vial of Castiel's blood can be seen sitting on Sam's desk. Crowley later comments that the Fallen Angel blood will likely be the trickiest ingredient to obtain and Sam and Dean don't correct him due to Crowley's hatred of Castiel.
After getting all of the ingredients, Sam and Dean mix the three bloods together and pour them over the bone of Sister Mary Constant. Nothing happens afterwards and they are left wondering whether or not the weapon will actually work. It does prove to be effective in the end, killing Dick Roman and ending the Leviathan threat.
- Season 5
- Season 6
- Season 7
- Season 9
- According to Crowley, Fallen Angel blood must be the hardest ingredient to obtain, but in actuality, it was the first and easiest component for the Winchesters to get. This was due to the fact that they knew Castiel and Crowley was unaware that the latter was still alive.