Matthew Mercer just surprised 'Critical Role' fans with another banger collaboration

• Matthew Mercer has a new collaborator on "Critical Role's" Campaign Three.
• Mercer has previously collabed with "Dimension 20" stars Brennan Lee Mulligan and Aabria Iyengar.
• But on Thursday night's episode, someone new — and unexpected — took over the reins as game master.

Caution: Spoilers for Critical Role Campaign 3, Episode 107.

"Critical Role " star game master Matthew Mercer has done it again.

For the third time in the ongoing nerdworld powerhouse's "Dungeons & Dragons" campaign, Mercer has given up control of the game to a guest star.

The CR crew — which has sold out arenas and branched out into ventures like two Amazon-backed animated series and making games of their own — are on the 107th episode of their ongoing "Dungeons & Dragons" campaign.

And in the last hour of Thursday night's episode, Mercer stepped aside and gave control of the game to Abubakar Salim of "House of the Dragon" fame.

Salim was one of the players on "Downfall," a CR campaign arc game-mastered by another guest game master, "Dimension 20's" Brennan Lee Mulligan .

Before "Downfall," Mercer had given way to Aabria Iyengar, known for her work on "Worlds Beyond Number" and "Dimension 20," for a quick whirl in the GM chair.

In "Downfall," Salim played the Arch Heart, a god in Mercer's world of Exandria.

On Thursday's episode, Salim reprised his role as the Arch Heart while game-mastering the final hour of the show. It was a campaign-defining moment and the Campaign Three characters' first extended interaction with a god of the Exandrian pantheon.

Mercer said during the episode's after-show cooldown that he and Salim, who's based in the UK, had been discussing the possibility of Salim coming back to CR since "Downfall" aired in July.

Salim then flew out to Los Angeles to film the episode. He and Mercer then had a two to three-hour crash course over lunch the day before they filmed the episode, where Mercer gave Salim all the lore he needed to take over the game.

"When Matt asked, essentially, I was like — yeah, I'll do it! And then I'm now backstage, like, 'what the fuck am I doing?'" Salim said on "CR Cooldown."

Salim's hour as game master also marks the first time Mercer has handed over the CR reins to someone outside his regular nerdworld collaborators. So all bets are off about who fans might see next on the show, especially as CR Campaign Three careens toward its final arcs.

On another front, the CR crew is also promoting the third season of their animated series, "The Legend of Vox Machina," which airs on Prime Video starting October 3.

From what we hear, it's going to be heartbreaking — in all the right ways.