According to Deadline, Ben Affleck and Matt Damon are set to team up once again, this time to be directed by Ridley Scott for a movie called The Last Duel. The movie will be based upon the longer-titled Eric Jager book: The Last Duel: A True Story of Trial by Combat in Medieval France.
That book is about the final officially recognized judicial duel that took place in France, which happened in the 14th century. The circumstances that led to that event happened after Norman knight Jean de Carrouges accused squire Jacques Le Gris of raping his wife. As such, the flick is set to be a revenge thriller, and while you can speculate about what role Damon will play and what role Affleck will play, it's set to remain just that (speculation) for now.
Deadline also adds that Carrouges and Gris are best friends, and that nobody believes the wife's version of events. Eventually the King of France decides that the two men fight and that the winner will be the one who is right, as it would be god's will. To add further drama (and maybe also to point out how stupid people were in the 1300s) if Carrouges loses then his wife will be burned at the stake for airing a false accusation.
Anyway, don't search for that book on Wikipedia if you don't want to don't want to know the ending.
Additionally, the movie will be penned by Damon and Affleck, as well as Oscar nominated writer Nicole Holofcener. That script is reportedly almost done, and will also have the distinction of being the first script to get made that both Damon and Affleck worked on, since 1997's Good Will Hunting. As you might know if you're old enough, Good Will Hunting was nominated for Best Picture at the Academy Awards.
Also, Scott last directed Damon in 2015's The Martian which, oh, was also nominated for Best Picture.