Warning! The following contains spoilers for The Flash episode "Memorabilia." Read at your own risk!
Just when audiences thought the reveal of Reverse-Flash was the biggest thing Season 5 of The Flash would do, The CW series upped the ante. "Memorabilia" featured another Batman reference, as a future interview with Captain Singh revealed an incarnation of The Dark Knight appeared in the Arrow-verse. This wasn't just any Batman though; this was the evil speedster version of the hero, a.k.a The Red Death. Now that we know he may arrive, we have a lot of questions.
Does Red Death Appear Before Or After "Crisis On Infinite Earths"?
As it stands, the most important thing coming up in the Arrow-verse is the 2019 crossover "Crisis On Infinite Earths." While it's not necessary to know whether Red Death will appear pre- or post-Crisis, finding out his arrival date could reveal how substantial his impact will be. For example, should the villain appear a month before the crossover, he may be gone by its conclusion.
The character's canon dictates he'd need Barry in order to gain control of the speedster's body. With that said, the Arrow-verse is known to take liberties with character origins, so it's possible Barry won't be the speedster The Red Death merges with to gain the powers of the Speed Force. Also, Season 5 has shown villains can still emerge even when timelines change, so perhaps The Red Death was a pre-crisis enemy that may surface post-crisis due to timeline altering.
Will This Be A Faithful Adaptation Of The Character?
As many probably already know, Red Death is traditionally the Batman of Earth -52, driven to extreme measures due to the amount of personal loss he's experienced. Desperate to stop crime, Batman uses the weapons of The Flash's Rogues to capture Barry. He then straps the speedster to his modified Batmobile, and hits the gas until he and Allen merge within the Speed Force. Thus, The Red Death is born.
Obviously, there are some problems with this interpretation when looking at the current state of the Arrow-verse. First off, audiences haven't even met a Bruce Wayne yet, let alone one from another world. This could mean another character is being eyed for the villain, which may lessen fan excitement for the villain substantially. Should the Arrow-verse really introduce this character if he's anyone other than Bruce Wayne?
Will This Tie Into Batwoman?
If a version of Bruce Wayne is coming into the Arrow-verse, one would think that Kate Kane would be interested in getting in on the action. After all, she has her own questions about what happened to her Earth's Bruce, so going toe to toe with a villainous version of him may provide some clues as to what's going on with the Bruce she knows. Plus, is there anyone better to psychoanalyze that villain's tactics?
There is, but he's M.I.A. at the moment. With that said, what if Batman is missing from Batwoman's Gotham City is because he's gone off the deep end and is working towards a plan to become The Flash? That'd be some intense foreshadowing from the Arrow-verse, and we're used to that by now. If DC television isn't allowed to use Batman, I'm completely fine with the Arrow-verse tarnishing the one Bruce we know of and making him Red Death.
Does This Mean Barry's Disappearing Again?
With the tease of Red Death comes yet another tease that Barry Allen will disappear from The Flash. In the comics, Batman merges with the body of Barry Allen, and while Barry's consciousness survives in the villain's brain, he is overtaken by the villain. This would seem to mean that even if Barry manages to stick around post-"Crisis On Infinite Earths," he might still disappear anyway thanks to the Red Death.
As mentioned though, the Arrow-verse loves to play with character origins. The Red Death could be created by a villain merging with any speedster, provided their suit is red so the name sticks. Jay Garrick is fair game, as is Jesse Quick. Either one isn't so important losing them would impact the show, and Barry could still run around and save the day. So it's definitely possible Barry may be absent when The Red Death appears, but not a guarantee.
Will Red Death Set The Stage For A New Crossover?
The appearance of Red Death in the Arrow-verse would be awesome, but imagine how awesome it would be if the next crossover is an adaptation of Dark Nights: Metal? For the uninitiated, the tale features Batman discovering a dark multiverse, and seven villainous copies of himself that include The Red Death. The story ends up involving many characters in the Justice League, which make it great for a crossover.
Not only that, it'd be a crossover extreme enough to top "Crisis On Infinite Earths." The already-announced Crisis could be the crossover to end all crossovers... unless of course the Arrow-verse is tasked with taking on a villainous Justice League of Batmen. That would truly be the crossover to end all crossovers. Would DC ever sign off on such a crazy premise? I'm skeptical, but at the same time insanely hopeful the answer is yes.
Does This Reveal Explain Bruce Wayne's Absence?
As previously mentioned, Bruce Wayne is missing in The Flash's Earth. While he might have gone cuckoo and is working out some elaborate plan to become The Flash, it's also possible he's into something even deeper than that. In Dark Nights: Metal, it's Bruce who discovers the existence of a Dark Multiverse, and ultimately opens the door that brings in the evil Batmen.
Someone working towards uncovering the existence of a Dark Multiverse wouldn't have much time to protect Gotham City, which may explain why his duties as protector have fallen by the wayside. This theory sounds crazy, although somewhat justified given The Red Death supposedly arrives on Earth at some point. Is this because Bruce found a door to the Dark Multiverse?