Dwayne 'The Rock' Johnson has signed on not just to play Black Adam in Shazam!, but to star in his own standalone movie as the villainous character. That sure is a lot of Dwayne Johnson.
The DC character Shazam (called Captain Marvel until a 1967 trademark conflict with Marvel Comics) is a boy who can transform into a superhero by speaking the word "Shazam." Black Adam, the role that Dwayne Johnson is set to fill, is Shazam's archenemy, a supervillain who occasionally acts as more of an antihero (making him sort of perfect for a standalone film).
Dwayne Johnson is certainly a superhero-sized actor, both in stature and in personality, so it'll be interesting to see him take on a more villainous role. The closest we've seen him to this is likely as Hobbs in his first appearance in the Fast & Furious franchise, before he graduated to a member of the team. If so, this is quite promising; he plays the imposing strongman character incredibly well.
So far, Johnson is the only star we know of; whoever's playing Shazam himself is either undecided or deeply under wraps at this point. That casting may prove critical to play off Johnson's Black Adam; Shazam is characteristically a classic square-jawed superhero, from a period of earnest, larger-than-life superheroes like Captain America and Superman, with Black Adam as his dark mirror. For the life of him, this writer can't think of any working actor that could fill this role opposite Dwayne Johnson, but perhaps New Line and DC can pull it off.
Shazam! is currently set for an April 2019 release date. The date for the Black Adam solo movie is as-yet unknown.