Castiel, pretending to be Bobby Singer tells Sam how it is described in a version of Revelations:

And he bloodied death under the newborn sky

Sweet to taste

But bitter once devoured

This is one of The 66 Seals which will be broken if two reapers are killed under a solstice moon. Castiel recruites Sam and Dean to prevent the seal from being broken, as the demons are holed up in a refuge protected by sigils that keep angels out. The boys are successful in stopping the seal being broken and in the process Castiel captures Alastair. Dean questions Castiel why, if death is part of the natural order, he was rescued from Hell. Castiel replies "You're different."