Since it was first launched in 2003, Robert Kirkman’s comic book series The Walking Dead has spawned a huge, media-spanning storytelling franchise that includes novels, video games, board games and – of course – the highest rated show in the history of cable television. AMC also created a series of webisodes that tied into the main series, and is now readying a spinoff series that has been given the working title Cobalt.

Although Cobalt is set in the same universe as The Walking Dead, it focuses on a completely different set of characters (though that doesn’t preclude the possibility of cameos). Earlier this year it was reported that the pilot episode would feature a group of six survivors, made up of two small families and one loner. Dave Erickson (Sons of Anarchy) is producing the series and veteran TV director Adam Davidson (Kingdom, Community) has been assigned to direct the pilot episode.

Now Deadline reports that Frank Dillane and Alycia Debnam Carey are the first cast members to join Cobalt. Based on the character descriptions it looks like they will be playing Nick and Ashley Tompkins, a brother and sister who are surviving the zombie apocalypse along with their mother, Nancy. Deadline‘s character details reinforce this, adding that Nick has battled a drug problem and Ashley is “ambitious,” which fits with the earlier description of her as being more level-headed than her brother.

Dillane is best known for playing a young Tom Riddle (AKA Voldemort) in Harry Potter and the Half-Blood Prince, while Carey starred in the natural disaster movie Into the Storm and recent horror film The Devil’s Hand. Both are relative newcomers, but the same was true of Emily Kinney and Chandler Riggs before they were cast in The Walking Dead.

The casting announcement also offers some new details about other characters that are yet to be cast. According to Deadline‘s report, the show will mainly revolve around Nancy Tompkins and Sean Cabrera, who were a guidance counsellor and a teacher (respectively) before the zombie apocalypse hit. In the previous character listings, Sean was described as “a good man trying to do right by everyone in his life,” while his son Cody is “whip-smart and rebellious” and known as “the angriest kid in town.”

It sounds like Nancy and Sean probably knew each other before everything went south, unless the fact that they both worked at a school is just coincidence. If that’s the case then Nick, Ashley and Cody could also be old schoolmates, though that doesn’t necessarily mean they’ll be friends.

We’ll keep you updated on Cobalt (or whatever it’s called) as development continues.