Speaking to E! Online, RZA revealed that Crowe will appear opposite his blacksmith character in the action film, which is set in feudal China.
He said: "I won't spoil it for you, but Russell's gonna be the baddest man alive. That man is in fighting shape. That man will knock you out."
RZA, who appeared with Crowe in Ridley Scott's American Gangster and also features in the actor's upcoming The Next Three Days, co-wrote the script for Iron First with Hostel's Eli Roth.