[S3E18] When The Levee Breaks
Throughout the fourth season, Lilith orchestrates the attacks by the forces of Hell on the 66 seals. They are opposed by angels, who resurrect Dean to assist them. Lilith eventually finds out that her death is the final seal that needs to be broken to free Lucifer. Reluctant to sacrifice herself, Lilith (Katherine Boecher) proposes a deal to Sam in "The Monster at the End of This Book": she will stop breaking the seals in exchange for his and Dean's lives. He rejects the deal and tries to kill her instead. She quickly overpowers him, but is forced to flee before she can do anymore when Dean tricks an archangel into coming to the location. In the season finale "Lucifer Rising", Sam kills Lilith under the impression that her death will prevent the final seal from breaking, and in doing so inadvertently breaks the final seal, releasing Lucifer.
[S3E18] When the Levee Breaks