Here is the official description for Castle season 8 episode 21, "Hell to pay" - airing May 9th.
Season 8 Episode 21 "Hell to Pay"
CASTLE BELIEVES THE ANTICHRIST COULD BE THE KILLER
When an axe-wielding, escaped psychiatric inmate drops dead in Castle’s P.I. office, Castle and Beckett’s investigation leads Castle to suspect the victim’s death could be the work of the Antichrist.
Nathan Fillion as Richard Castle
Stana Katic as NYPD Captain Kate Beckett
Tamala Jones as Medical Examiner Lanie Parish
Jon Huertas as NYPD Detective Javier Esposito
Seamus Dever as NYPD Detective Kevin Ryan
Molly Quinn as Alexis Castle
Susan Sullivan as Martha Rodgers
Toks Olagundoye as Hayley
guest & recurring cast:
Ronnie Steadman as Gabriel Shaw
Emanuel Loarca as Guard
Michael Chieffo as Dr. Brock Martin
Anne-Marie Johnson as Sondra Kramer
Matthew Borlenghi as Rollins