Warning: The following contains SPOILERS for The Flash season 8.
It appears The Flash season 8 has set up a major Green Lantern event from the comics, known as the Blackest Night. Originally published in 2009-2010, Blackest Night was the culmination of several storylines in the Green Lantern comics of the time, but it grew to engulf the whole DC Comics universe. While being a well-regarded event, Blackest Night remained a rather left-field choice to be adapted for the Arrowverse until the introduction of the undead villain Deathstorm.
The second major story arc of The Flash season 8 centered around a mysterious metahuman murderer, whom Dr. Caitlin Snow (Danielle Panabaker) came to believe was her dead husband, Ronnie Raymond (Robbie Amell), confused and transmuted into a new form. The rest of Team Flash was skeptical, despite Caitlin determining that the only common link between the victims, apart from how they died, was that they were all people she knew. Unfortunately, Caitlin's faith proved to be ill-justified, and her efforts to bring her husband back to life only served to give the Black Fire Killer a more tangible form. Referring to himself as Deathstorm, the Black Fire Killer revealed his true nature to Caitlin at the end of The Flash season 8 episode «Resurrection» and that Ronnie Raymond's return as Firestorm was a ruse.
Related: Did The Flash Really Just Introduce DC's Undead Firestorm?
A villainous being with Ronnie Raymond's memories was one of the many monsters created as part of the Blackest Night event in the comics, in which the death god Nekkron sought to take over the universe by raising an army of undead superheroes and supervillains. Later, during the Brightest Day story, which followed Blackest
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