The presence of Hayden is another bit of information to suggest that the Arrowverse is gearing up for a version of Crisis on Infinite Earths, DC's biggest and most impactful crossover of all time. That storyline featured The Monitor and The Anti-Monitor, cosmic twins who oversaw the matter and anti-matter universes at the beginning of time. Ultimately the Anti-Monitor destroyed most of DC's multiverse, leading to all of the company's various timelines being merged into a single universe and a single continuity. That status quo remained in place, with the occasional tweak, for about twenty years.
The Flash cited the Crisis in its series premiere, with a newspaper from the future appearing that said Barry would vanish, along with red skies that had been looming over Central City, at the end of a "Crisis." In last month's season 5 premiere, the future was changed somewhat, along with the newspaper article, which now mentioned Hayden/Psycho-Pirate.
“Research has focused on Roger Hayden, a.k.a. Psycho Pirate, who claims to remember the night’s events,” the article reads in its current state. “He said upon his arrest, ‘Worlds lived, worlds died. Nothing will ever be the same.'”
Frazer had a bit part in an episode of Supergirl last year, as evidenced by his profile picture and header graphic on Twitter, which feature photos of him with Kevin Smith, Melissa Benoist, Katie McGrath, and Odette Annable.
LaMonica Garrett has already been confirmed to appear in the crossover as the Monitor. He, along with Psycho-Pirate, is one of a handful of key characters who drove the plot of Crisis.
Hayden's powers came from a "Medusa Mask," which he could use to project emotion onto others. Given that The Flash has been dealing with meta-objects this season -- objects imbued with dark matter and super powers in the same way that people have been in past seasons -- Hayden's mask could presumably come into play this season on The Flash as part of setting up the Crisis.
The crossover will also feature guest appearances by Ruby Rose as Batwoman, Tyler Hoechlin as Superman, Elizabeth Tulloch as Lois Lane, Cassandra Jean Amell as Nora Fries, and Jeremy Davies as John Deegan, an Arkham scientist who is likely tied to Doctor Destiny.
The Arrowverse crossover, "Elseworlds", will take place across three nights, kicking off on Sunday, December 9th on The Flash at 8 p.m. ET. It continues on Arrow Monday, December 10th at 8 p.m. and then finishes up on Supergirl Tuesday, December 11th at 8 p.m. ET.