Since we get 23 episodes of Supernatural this year, the May 11 episode is actually the next-to-last one of the season!
Here's the official description courtesy of The CW.
SAM AND DEAN SEEK OUT AN ALPHA --- In order to defeat Dick Roman (guest star James Patrick Stuart) and the Leviathans, Sam (Jared Padalecki) and Dean (Jensen Ackles) must locate three key items. Castiel (Misha Collins) helps with one but for the last objects, the Winchesters must face two of their strongest opponents - an Alpha (guest star Rick Worthy) and Crowley (guest star Mark Sheppard). Meanwhile, Bobby (guest star Jim Beaver) tries out a new ghost trick that ends up being very dangerous. Guy Bee directed the episode written by Andrew Dabb & Daniel Loflin (#721).