The phrase "impending apocalypse" means that a destructive event or disaster is about to happen soon, which could lead to widespread devastation and the end of the world as we know it. Full definition
A sense of impending apocalypse hangs heavily over the whole film. (vox.com)
As a director, Nichols creates such an intense aura of dread and impending apocalypse during the visions that when Curtis simply describes one that is not shown in the film, we shudder at the mental image it paints. (rottentomatoes.com)
About 1 millennium later, in the approach to the current millennium, a number of people became wealthy enough writing and lecturing about another supposedly impending apocalypse: the supposed Y2K problem. (slaw.ca)