This isn't Hollywood's first rodeo in the grim darkness of the 41st Millennium. Three years ago Games Workshop agreed to option out the Eisenhorn series of novels by Dan Abnett with a view to developing a TV series to be helmed by Man in the High Castle and X-Files producer Frank Spotnitz. It is believed that Spotnitz held discussions with Amazon, whom he worked with on High Castle, but the project did not move forwards at that time.
Eisenhorn is a reasonable starting point for the franchise, with a cast consisting of mostly human characters with only occasional appearances by the Space Marines (eight-foot-tall, genetically-engineered warriors who are the signature faction of the setting) and daemonic forces. This is an easier entry point versus the total gonzoid epic war insanity of something like The Horus Heresy series.
It is also possible Amazon might look to develop a series based on Abnett's Gaunt's Ghosts series, which features relatable characters belonging to the Imperium's regular human army, the Imperial Guard. However, both Amazon and Games Workshop may be keener for something that front-and-centres the Space Marines and other key factions like the Orks, Tyranids or Necrons.
Cavill is a noted Warhammer franchise fan. He's appeared in videos to discuss the lore and his love of painting Warhammer miniature figures, and spoken of his appreciation for several of the spin-off video games and novels. He has even corrected confused interviewers over the differences between the Warhammer and WarCraft universes.
Games Workshop, Amazon Studios and Vertigo Entertainment will collaborate on the first project, the details of which have yet to be revealed, with Cavill tapped to star and executive produce, as well as extending his advice over other projects in the franchise. Games Workshop and Cavill both appear to be keen for any adaptation to hew close to the source material and not deviate purposelessly away, which seemed to be a bone of contention between Cavill and Netflix over their work on The Witcher.
This project is very early in development.