[s4e21] When The Levee Breaks -

: The title is a nod to a 1929 blues song famously covered by Led Zeppelin in 1971.

: Dean tracks Sam to a motel, leading to a violent brawl. After Sam nearly strangles him, Dean delivers a final ultimatum: if Sam walks out the door, he should never come back. Sam leaves, effectively severing their bond as they head into the finale. Major Character Dynamics [S4E21] When the Levee Breaks

: Chooses to swear an oath of fealty to God and the angels, believing it is the only way to save Sam from becoming a monster. : The title is a nod to a

: Ruby reveals that Lilith is "Lucifer's first" demon and that only she can break the final 66th seal. [S4E21] When the Levee Breaks