Before Castle delves into the series-long LokSat mystery to wrap up its eight season, the crime-comedy returns on Monday with yet another standalone episode that deals with the Antichrist mystery. Episode 21 appropriately titled Hell To Pay will see the author nursing a rather silly belief that he is cursed. While Kate Beckett plays on her partner's crazy belief, she has to solve the mystery related to the death of a psychiatric inmate. The episode airs on ABC on 9 May at 10pm EST.

The official synopsis of the episode reads:

If the trailer for the episode is any indication, it sure is going to be one hilarious episode. As one event after another unfolds, Castle is led into a bizarre thought of him being cursed. However, the mystery at hand – the death of an "axe wielding psychopath" – leads Beckett and Castle's investigations towards an interesting point where Rick believes that the victim's death could be the work of the Antichrist.

Watch the trailer here:

The trailer opens with two visibly shocked women as a man breaks open through the door; soon enough Castle harps on his string of theories, "it's an omen," he continues to add, "I am being targeted by dark forces." Cursed or not, Beckett sure has a fun time fuelling his fear with pranks of her own.

Talking about the pair, it seems to be one of the last times Kate and Rick work together to solve mysteries. With the season finale just a week away, where rather serious investigations take place and the plot prepares to bid goodbye to Stana Katic's character, it also appears that the penultimate episode would be playing on humour for a final time.

As the show closes its finale, fans can't help but wonder how Katic's character would be written off the show, with rumours suggesting that Beckett might die in the finale. In the meantime, TV Line reported that ABC will return with season 9 of Castle along with Nathan Fillion and Seamus Dever. While Katic's farewell was long-known, it seems like Castle-creators will write off another character played by Tamala Jones. "We are grateful for Stana Katic's talent and dedication to the series and we hope to continue our relationship. Tamala Jones has also been an integral part of the series and we are grateful she was a part of Castle," said an ABC statement.

Click here to watch the episode via live stream on ABC, or watch it online on ABC's official website in the US.