The Winchesters seem to have caught a break in their quest to remove the Mark of Cain from Dean's arm when Charlie (Felicia Day) calls them to say she's found the Book Of The Damned, a legendary tome of the black arts that supposedly contains the spell they are looking for.
Unfortunately, the book is also being sought by its previous owners, an evil family of magicians headed by Jacob Styne (Jeff Branson).
Charlie has been wounded in a run-in with the Styne's, so Dean and Sam meet her at one of Bobby's old hunting shacks, where they try to translate the book - which turns out to have been written in blood by a mad nun, who used slices of her own skin to make the pages. On top of that not only did the nun write the book in a dead language, but also in code!
Meanwhile, the episode's B storyline sees Castiel and Metatron on a road trip to recover Castiel's stolen Grace, only - surprise, surprise - Metatron, while enjoying his current 'human' condition, is not to be trusted.
For a large part, Book Of The Damned felt like yet another bridging episode, getting the characters from where they were to where they needed to be for the next (final?) part of the season arc.
However, where it scored was in the bait-and-switch finale and Sam's unexpected decision in the coda, which simply demonstrated how desperate he is to save his brother's soul.
The Styne family - what we learned of them - seemed like interesting villains and I hope the show actually does something with them and doesn't just forget about them, as it did with the (very similar) Thule Society from Everybody Hates Hitler.
You know, maybe, the showrunners have been playing a long game and the Stynes and the Thule Society are one-and-the-same? Sam did reference events from the pilot episode (all those years ago) during his conversation with Charlie about the life of a Hunter. This gives me hope that the writers will continue to draw upon the established mythology of the Supernatural 'Verse as well as introducing new elements.