Dark Nights: Metal: Dark Knights Rising (2017): written by Scott Snyder, Grant Morrison, James Tynion IV, Joshua Williamson, Frank Tieri, Sam Humphries, Dan Abnett, and Peter J. Tomasi; illustrated by Doug Mahnke, Philip Tan, Tony S. Daniel, Francis Manapul, and others: There's no point reading this as a separate entity from the DC Comics Event series it supports, Dark Nights: Metal. Well, unless you like depressing What if? superhero stories about horrible alternate realities in which Batman goes crazy and kills off most or all of the other super-heroes. Then it's awesome!
I think you're supposed to read these stories about halfway through your reading of the main series, but reading them afterward (as I did) worked fine. They fill in some of the blanks of the main series. The writing is mostly solid and occasionally inspired. The art, too, is solid and occasionally inspired. The Seven Evil Batmen from alternate universes are depressing and awful here as in the main series. Moreso, really, as we see the depth of their falls from grace. Recommended.

Dark Nights: Metal: The Resistance (2017): written by Scott Snyder, Benjamin Percy, James Tynion IV, Joshua Williamson, Rob Williams, Robert Venditti, and Tim Seeley; illustrated by Doug Mahnke, Howard Porter, Yanick Paquette, Jorge Jiminez, Jaime Mendoza, Liam Sharp, and others: There's no point reading this as a separate entity from the DC Comics Event series it supports, Dark Nights: Metal. You are supposed to read the stories collected here about four issues (of a total of six) into Dark Nights: Metal. Certain major things in that series are explored and explained. To say more would spoil that.
But yeah, you're going to need to read Dark Nights: Metal. And Dark Nights: The Road to Metal if you want to understand why Dick Grayson/the original Robin/Nightwing keeps having transdimensional visions.
Well, no, not exactly. Perhaps 2/3 of the volume comprises The Resistance. That follows the efforts of the Bat-Family, the Suicide Squad, and Green Arrow to figure out what the Hell is going on in Gotham City in the absence of the (real) Batman. The Seven Evil Batman are there. So too is Challengers Mountain, dropped into the middle of Gotham like a lawn dart made of sweet, sweet granite.
So, too, concentric circles patrolled by the minions of the Dark Batmen. Harley Quinn and Killer Croc get a lot of space to be their good-bad selves, fighting for Gotham even though they're mostly villains. It's probably about as fun as it can be, though it refers to events preceding the story-line that aren't collected in any of the four Metal volumes. Oh, well. Recommended.
Dark Nights: Metal (2017-2018): written by Scott Snyder with James Tynion IV; illustrated by Greg Capullo, Mikel Janin, Alvaro Martinez, Jonathan Glapion, Raul Fernandez, and others: Metal writer Scott Snyder notes in his foreword that he wanted this Event Series to be a big event like the ones he remembered enjoying in his youth. And Snyder does manage lots of cosmic melodrama, dire moments, and seemingly doomed heroic final stands.
Metal may have the oddest set-up for a cosmic event comic ever. In the months prior to Metal, Batman had been investigating the origins of the weird metals of the DC Universe. That would include the resurrectional Electrum of his enemy The Court of Owls, the strange Nth metal of Hawkman's mace and wings, and even the protean shapeshifting of Plastic Man himself.
Against all advice, Batman -- who has probably been the cause of and solution to all of the Justice League's problems more than any other hero -- pursues his quest to the point of fulfilling an ancient prophecy that he thought he was working to forestall. Hoo ha!
To not give anything away, Batman's successful failure allows a whole lot of bad things to invade the DC Universe. It will be up to Batman, Superman, Wonder Woman, and all Earth's other heroes to rescue the multiverse from Batman's mistake.
Snyder proves to have a strain of cosmic goofiness in him that I was not aware of. Metal evokes the original craziness of DC's 1950's and 1960's Silver Age while also playing at the edges of metafictionality as do the cosmic DC Comics of Grant Morrison. This is a story that is very explicitly about Story. Bringing Daniel, the 'new' Lord of Dreams (well, new since the conclusion of Neil Gaiman's Sandman back in 1995) into the fray serves to make the whole Story emphasis very, very emphatic.
It's not much of a stretch to note that essentially the DC Multiverse comes under fire from a whole lot of misguided pro revisionism and creepy fan fiction. I kid you not.
It all works, somehow. Greg Capullo, who partnered with Snyder on a lengthy Batman run, channels his days drawing cosmic melodrama on Todd Macfarlane's Spawn to good effect. Things get a bit crowded with characters, not really a problem because that too is a nod to George Perez's meticulous, overcrowded work on the Nexus of all DC Comics Event Series, Crisis On Infinite Earths. Capullo does a nice job with all the punching and the kicking, the weird character designs for the invading villains, and the endless leagues of heroes and villains he must draw.
Metal certainly isn't perfect. Like most Event Series, a number of story points briefly touched upon in the main narrative require the purchase of other comics in which those points are fleshed out more fully. Things get a little rushed at the end, to the extent that some confusion sets in as to who is doing what where, and what the heck is happening in some of the action sequences. This is not a problem peculiar to Metal. But in all, this is an enjoyable superhero comic that could probably be read by someone who's not fluent in the 80 year history of DC superheroes. Recommended.
Dark Days: The Road to Metal (2018): written by Scott Snyder, James Tynion IV, Grant Morrison, and Tim Seeley; illustrated by Andy Kubert, Jim Lee, John Romita Jr., Doug Mahnke, J.G. Jones, Eddy Barrows, Chris Sprouse, Greg Capullo, and others:
Technically a hybrid collection consisting of the prologue to the Batman/JLA story Dark Days: Metal, that prologue being Dark Days: The Forge and Dark Days: The Casting, along with select stories that supply some background to Dark Days: Metal. In theory, this volume could have been about a gajillion pages long.
Why?
Because Dark Days: Metal attempts to create a Grand Unified Theory for the origin of a wide variety of the strange metals and substances that have been appearing in DC Comics since 1940, from the original Hawkman's anti-gravity Nth Metal to the resurrecting Lazarus Pit of Batman foe Ra's Al Ghul. And dozens more. And the theory brings in the origin of certain types of superpowers, the reason why the Joker never dies, and... well, a lot.
Dark Days: Metal writer Scott Snyder is definitely better at this sort of continuity epic than most, in part because the prologue is structured as a detective story. Batman tries to unravel the mystery of a variety of signs and portents linked to the mysterious origins of a whole lot of weird stuff in the DC Universe.
Not to pick a nit too much, but the volume could actually use one of those Who's Who encyclopedia-like stretches in which the various thingies we see and are occasionally informed about are simply listed in terms of name, appearance, that sort of thing. I know that the 'Nth metal' of Hawkman's wings dates back to his first appearance. I'm a little shakier on '8th metal.' I know the Lazarus Pit from 50 years of Batman stories. The Electrum Ring of the Court of Owls? Unh, no.
Or was Hawkman's mace made of Nth metal? See what I mean?
But as a prologue, this is certainly enjoyable stuff with some nice art and an interesting selection of earlier stories that also seem to have been selected to force you to buy those stories in their entirety, as most are cliffhangers from longer story arcs.
In order to create even more cliffhangers, I'd recommend a volume consisting of pages or even panels introducing various objects and substances, ripped without context from 80 years of DC Comics. Recommended for continuity hounds. Maybe not so much for people who don't know what Nth Metal, a Lazarus Pit, or Vandal Savage are.