'Dark Nights: Metal' #6 Features an Appearance by SPOILER

Spoilers ahead for Dark Knights: Metal #6, on sale now.

In one of the decisive moments of Dark Nights: Metal, the crossover series from acclaimed Batman creative team Scott Snyder and Greg Capullo, an army of Batmen from throughout DC's expansive multiverse swooped in -- including at least one face that surprised fans:

That of Frank Miller's bulky, aging Batman from The Dark Knight Returns.

The story, told in an alternate future where a long-retired and borderling psychotic Batman comes out of retirement, is one of DC's best-selling collected editions of all time and is widely regarded as one of the best Batman tales ever. A pair of sequels (The Dark Knight Rises and Dark Knight III: The Master Race) were less well-received but still huge sellers.

The "Dark Knight" Batman has been absent from nearly all of the various multiverse-spanning stories DC has told over the years, and so his arrival -- like that of Sandman before him -- was somewhat unexpected for longtime fans.

Metal added the idea of a Dark Multiverse to the DC orrery of worlds -- a multiverse made up of dark matter and populated by the stuff of nightmares, it provided the story with its Dark Knights, twisted reflections of Batman from throughout the dark multiverse.

It should be little surprise, then, that just when those Dark Knights appeared to have won, the multiverse's salvation would come in the form of an army of GOOD Batmen.

The appearance was little more than fanservice; in the crowded, bombastic finale, no single character had a ton to do, so it served primarily to excite fans when they saw the image of a beloved alternate version of the Dark Knight rushing in to help.

Besides the two existing sequels and a stand-alone one-shot that dealt with the Joker of the Dark Knight Returns world, Miller and collaborators are working on other Dark Knight-related stories, including an upcoming original graphic novel that centers on Carrie Kelley, the young, female Robin of the Dark Knight universe.

There is much more to break down in Metal's finale, so check back tomorrow for some of that.

Dark Nights: Metal #6 is in stores now. You can get digital copies at ComiXology, Barnes & Noble, and Amazon.