It's a Rannian legend. The hero Katarthul found an artifact capable of reopening a doorway to another dimension. He hid it in a place no one would find it. Because if the key were ever activated it would draw the life energy from the nearest worlds to power itself. 
Hawkman (Volume 5) #21 is an issue of the series Hawkman (Volume 5) with a cover date of April, 2020. It was published on February 12, 2020.
Synopsis for "Death's Doorway Part Two: The Key"
Appearing in "Death's Doorway Part Two: The Key"
Featured Characters:
- Carter Hall (Body Possessed) (As a spirit)
Supporting Characters:
- Adam Strange
- The Atom (Ray Palmer)
- Hawkwoman (Shayera Tal)
- Paladin of Titan Hawk (Real name revealed)
Antagonists:
- Sky Tyrant (Earth 3) (In Carter Hall's body)
Other Characters:
- Titan Hawk (Deceased) (Statue only) (Cameo)
- Dread Clergy (Mentioned only)
- Lord Beyond the Void (Mentioned only)
- Paladins (Mentioned only)
- Paladin's father (Unnamed) (Deceased) (Mentioned only)
- Rannians (Mentioned only)
- Katarthul (Deceased) (Mentioned only)
Locations:
- Multiverse (Behind the scenes)
- Earth 0
- Titan Hawk's Planet (Unnamed)
- Hyperspace
- Earth 0
- Dark Multiverse (Mentioned only)
- The Lord Beyond the Void's Realm (Unnamed) (Mentioned only)
Items:
- Bio-Belt
- Dark Matter (Behind the scenes)
- Key to the Realm Beyond the Void (First appearance) (Cameo)
- Nth Metal
- Sky Tyrant's Mace
- Titan Hawk's Mace (Destroyed)
- Rannian Jet Pack
- Zeta-Beam
Vehicles:
- Hawkwoman's Thanagarian Cruiser
- Shrinkship
- The Soarship
Events:
- Justice/Doom War (Behind the scenes)
- Year of the Villain
- The Infection
Concepts:
- Dimensional Doorway (Mentioned only)
- Magic
- Metahumans
- Reincarnation
See Also
Links and References
Look at how sad this is making Batman. You did this.