Critical Role's 'Downfall' is a new crossover project for the 'Dungeons & Dragons' history books

• Critical Role's new campaign arc, "Downfall," will be helmed by "Dimension 20's" Brennan Lee Mulligan.
• The three-chapter series is all about the fall of Aeor, an ancient civilization.

It looks like "Critical Role" is venturing into uncharted nerd world territory again, and it's bringing in the big guns.

The eight-member CR cast is on episode 98 of their third long-running "Dungeons & Dragons" campaign. For the uninitiated, this group has spent nine years creating everything from two Amazon-backed animated series to a board games publishing arm.

"Critical Role" is no stranger to guests taking over for the gang's regular game master, Matthew Mercer. The crew is now doubling down on experimentation and bringing game master Brennan Lee Mulligan to helm a full campaign arc. Mulligan is the regular game master for "Dimension 20," a tabletop role-playing game show that streams on Dropout, formerly CollegeHumor.

Titled "Downfall," the three-part series goes back in time to the fall of an ancient civilization, Aeor. It'll also allow Mulligan to flex his storytelling chops, perhaps reprising some elements of his first "EXU: Calamity" run with Critical Role from 2022.

The three chapters are integrated into "Critical Role's" main campaign as episodes 99 to 101 of the main storyline. Only three members of the regular CR cast — Laura Bailey, Taliesin Jaffe, and Ashley Johnson — will be playing in "Downfall." They'll be joined by a new slate of guests, including Abubakar Salim, Noshir Dalal, and Nick Marini.

Mulligan — whose own crew is scheduled to play "Dungeons & Dragons" at a sold-out Madison Square Garden in January — told this writer in May that he was excited about the "delightful" prospect of getting to work with "Critical Role" again. At the time, Mulligan had only been billed as an emcee for "Critical Role's" June live show.

"Without being overly sappy or sentimental about 'Critical Role' and its place in the pantheon of actual play: This was like 2017, and I'd been working at CollegeHumor for a couple of months," Mulligan recalled, referencing "Dimension 20's" beginnings as a CollegeHumor show. "And I came in with this little market research document of people that are doing work in this space that is exceptional and phenomenal, like 'Adventure Zone' and 'Critical Role.'"

"Maybe no one would have taken a chance on "Dimension 20" if the path forward was not paved by the incredible work of Matt Mercer and the whole gang over at CR, and it's just so exceptional," Mulligan added. "I just have so much respect and appreciation to even be a little, tiny part of that incredible world they've built."

The "Downfall" episodes drop on July 11, 18, and 25 on Twitch, YouTube, and CR's in-house subscription service, Beacon.