Alan Moore and Dave Gibbons’ self-contained twelve-issue comic Watchmen was released by DC Comics in 1986-1987, though it takes place in a completely separate fictional universe from their mainline characters. It’s been pretty universally praised since its release, but was considered “unfilmable” for a long time, having been so described both by its writer Alan Moore and by putative director Terry Gilliam.
So it proved through a long series of failed attempts to make a film version. But it was finally done in 2009, in a film directed by Zack Snyder and featuring a mostly unknown cast — an excellent decision to my mind. The last thing we needed was be constantly thinking “Oh, look, Johnny Depp” when we should be seeing The Comedian.