Speaking at a private press event during C2E2, DC co-publisher Jim Lee revealed that the "metal" in the title of DC's summer event Dark Knights: Metal is none other than Nth metal - the mysterious Thanagarian element that allows Hawkman to fly and has other strange properties.
Nth Metal has undergone a few different iterations in DC Comics. Described as a "heavy iron isotope" with the periodic designation Fe676, Nth Metal has psycho-reactive properties that can allow flight, the storage of memories and psychich imprints, and even resurrection.
In one version of Hawkman's backstory, the ancestors of Carter and Sheira Hall are Thanagarian visitors to ancient Egypt. Their murder with a knife of Nth Metal sparks the longspanning series of reincarnations and resurrections that lead to the modern day Hawkman and Hawkgirl.
Nth Metal is also used in the Legion of Super-Heroes famous flight rings, in Deathstroke's armor, and elsewhere.
Hawkman has appeared during "Rebirth"in the Death of Hawkman mini-series, which writer Mark Andreyko promised Newarama would expand to later "Rebirth" stories despite the ominous title. Hawkman was specifically mentioned in the solicitation for Dark Days: The Casting #1, one of the stories directly leading to Dark Nights: Metal.
Dark Nights: Metal #1 is due out in August.