For fans of Stephen King, there's no more important work from the author than The Dark Tower. Considered King's Magnum Opus, the sprawling series spans eight novels, several comic books, and crosses over into a number of King's other stories, making it the centerpiece of the entire King Universe. Now, it's being adapted into a film — The Dark Tower — which will pit the last gunslinger against the evil Man in Black. But what happened to the gunslingers, and why is there only one left?
Idris Elba portrays Roland Deschain in the film, the last gunslinger of Mid-World, the alternate dimension that is his world. The gunslingers were an order of knights armed with six-shooters and tasked with keeping law and order in Mid-World. Now, Roland is the only one remaining, and he is on a quest to reach the Dark Tower — the center of all realities — to save his world from total collapse. An evil sorcerer called The Man in Black (Matthew McConaughey) is also on a quest to reach the tower, so that he can rule over all realities. That's the main gist of the movie, but to find out what happened to the gunslingers, you've got to go back a little further.
The gunslingers all died in a war that left Mid-World in the crippled state it's in now, with Roland being the only one from their ranks to survive. A rebellion of Mid-World's lower-class citizens, led by a man named John Farson, decimated the gunslingers thanks to their vast army of barbarians and mutants. The final decisive blow took place at the Battle of Jericho (as depicted in the comic of the same name), where the Man in Black — under the alias of Farson's general Walter O'Dim — led an attack that wiped out the last of the gunslingers save for Roland, whom O'Dim didn't realize had survived until later.
The movie doesn't delve into this backstory too much, as it's more focused on Roland's current quest to reach the Dark Tower. However, a 10 to 13 episode TV series is going to follow the first movie and expand on Roland's backstory, which means fans could potentially end up seeing the fate of the gunslingers unfold on TV in a couple years.