S5, E21: Crowley (guest star Mark Sheppard) tells Bobby he will give him the location of Death (guest star Julian Richings), the fourth horseman, in exchange for his soul. Knowing Sam and Dean need that fourth ring to stop the Apocalypse, Bobby reluctantly agrees. Sam and Dean confront Pestilence (guest star Matt Frewer), but he unleashes a deadly virus upon them – so Castiel must intervene on their behalf. Dean has a meeting with Death to discuss Lucifer, and an unholy alliance is formed at a very high price for Dean.