The latest episode of Supernatural featured meta jokes on top of meta jokes, so it stands to reason that the title of the episode might also include some hidden secrets. Fans were quick to notice that The Monster at the End of This Book is actually the title of a Sesame Street children's book featuring Grover. Could this harmless piece of kiddie lit hold the secret to the entire Supernatural mystery?
In the book, which is itself a piece of meta fiction, Grover talks directly to the person reading the book, warning them about the monster at the end and urging the reader not to continue reading. It's a rather adult concept, but if you reach the end, you get a surprise.
SPOILER WARNING: Read no further if you don't want the end of this children's book spoiled.
At the end of The Monster at the End of This Book, it's revealed that the monster is actually Grover himself and was all along. Though Grover never realized it, he is the monster. But how does this factor into Supernatural?
Applying the book's ending to this season of Supernatural, maybe the monster of the season has been there all along. The monster is Lucifer, who Lilith revealed is trapped in a cage. If the cage is just metaphoric, maybe Lucifer is actually lying dormant in a person. Maybe Lucifer is Sam Winchester.
Before I'm burned at the stake for suggesting such an idea, there may be a sound basis for this theory. The breaking of the 66 Seals this season has mirrored Sam's own descent into darkness, drinking demon blood and becoming more powerful.
In addition, Lilith wanted to make a deal to stop breaking the Seals in exchange for Sam's head on a spike. If she has seen the end and knows that she won't make it to see the good part, maybe she knows that when the final Seal is broken, Lucifer will take over Sam's body and kill her, fulfilling Sam's own promise to be the one to kill Lilith. It could also explain the angels trepidation over Sam using his powers.
Obviously letting Sam be overtaken by Lucifer would be a controversial move, but as Chuck Shurley said, he does put Sam through plenty of suffering for the sake of artistic symmetry. A life that started with a demon dripping blood into the baby's mouth as part of a deal is one that could end with the ultimate revelation that the entire deal was designed to bring forth Lucifer.
There's also some beautiful artistic symmetry if Dean is the one chosen by God and his own brother is Lucifer's vessel. That's the kind of storytelling Chuck Shurley would enjoy.