The directors of 'Avengers: Endgame' and 'The Gray Man' are making a spy thriller series for Amazon that has reportedly become the second-most expensive show of all time

• Joe and Anthony Russo are making a spy thriller for Amazon called "Citadel."
• The budget has ballooned past $200 million, according to The Hollywood Reporter.

Joe and Anthony Russo, the directors of Marvel's "Avengers: Endgame" and Netflix's "The Gray Man," are making a spy thriller for Amazon that has become one of the most expensive TV shows ever made.

The Hollywood Reporter reported on Thursday that the budget for the series, "Citadel," has inflated beyond $200 million due to pandemic-related expenses and costly reshoots.

That would make it the second-most expensive season of a show ever — behind Amazon's own "Lord of the Rings: The Rings of Power," which debuted its first two episodes on Thursday. The rights for that series alone cost $250 million and the production costs for season one reached $465 million, according to THR.

Representatives for Amazon Studios and the Russos did not immediately returns Insider's requests for comment. They declined to comment for THR.

"Citadel" will bring together spies from across the world for a single mission, with the intention of launching local spinoff shows focused on different spies in their own territory.

The Russos have been developing the series for some time. In an interview with Insider in 2018, they said that it provided a "new concept for storytelling."

"The concept with this show is international reach, and how to tell a global story," Joe told Insider at the time. "There will be a flagship show and some subsidiary shows that are set in foreign language markets."

"It plays very much to the Marvel movies we've been making, which have such a global reach with passionate audiences around the world," Anthony added. "We've been spending the last couple years interacting with those audiences. We grew up on global cinema and we love global cinema. I think the idea of communicating with a worldwide audience has always been appealing to us and now we're doing it on a larger and larger scale."

But the series has been plagued by behind-the-scenes drama, according to THR. Josh Appelbaum, its original showrunner, was fired from the series in December by Amazon over creative differences with the Russos, prompting others to depart, including director Brian Kirk, THR reported.

The Russos have been afforded large budgets and creative freedom in recent years thanks to their Marvel success. "The Gray Man," which is now Netflix's fourth biggest movie ever, cost $200 million to make.