Dark Nights: Metal introduced quite a few new (and pretty awesome) characters into the DC Universe, one of which is Red Death (created by Scott Snyder and Greg Capullo). Metal was one of the more recent DC events, so, for those who may be a bit behind, Red Death probably isn't a very familiar character. He has a pretty awesome story, and there's quite a bit of information to go through to get caught up, so let's get right into it.
Earth -52
Let's take it from the top. In the alternate reality Earth -52, Batman is still a crime-fighter. As occasionally broken and dejected as the Batman we all know and love is on our main view into the DC Universe, the Batman from Earth -52 became even more shattered by trauma. He fought alongside many different Robins, but many of them fell prey to the murderous hands of -52 Batman's own Rogues Gallery. After outliving yet another Robin, -52 Batman became obsessed with finding more extreme, and more effective, ways of fighting the crime that plagued -52 Gotham. There were too many crimes, too many disasters that he wasn't quick enough to save people from. He needed to be everywhere at once, he needed to be faster, he needed...
Fighting -52 Flash
-52 Batman decided the only way to save his Gotham, to save his world from the festering forces of destruction that plagued mankind, was to obtain the powers of The Flash. -52 Batman acquired the weapons of many of -52 Flash's Rogues Gallery, including Captain Cold's Freeze Ray, Heat Wave's Heat Gun, Weather Wizard's Weather Wand, and Mirror Master's Mirror Gun, and with all of those at his disposal, he bested -52's Flash, after that Flash refused to surrender his powers to him.
Merging Into One
-52 Batman rendered The Flash unconscious and chained him to the hood of the Batmobile, which -52 Batman modified after obtaining designs for the Cosmic Treadmill. -52 Batman drove both himself and -52 Flash into the Speed Force with the Batmobile, and the event merged them together, giving Batman control of the Speed Force and trapping -52 Barry Allen inside that Batman's body. "My name is Bruce Wayne. I am vengeance. I am Justice. I am Batman: the Red Death."
Newfound Power
Now that he'd become Red Death, many of the villains in Gotham stood little chance. With his newfound powers, Red Death murdered the entirety of his Rogues gallery in a misguided attempt at saving his planet. Shortly after, he realized that even with his Rogues gone, and even with the power of The Flash, it was too late to save his planet...
The Batman Who Laughs
...until he was visited by another alternate version of Batman: The Batman Who Laughs. The Batman Who Laughs tells Red Death that he can still save his planet if he joins Barbatos' Dark Knights in their plot to conquer a multiverse. Red Death accepted The Batman Who Laughs' invitation and traveled with him and the rest of the Dark Knights to the version of the DC Universe we're all most familiar with.
Infecting Central City
Upon entering our own window into the DC Universe, The Red Death attempted to kill The Flash. Red Death gained the upper hand in his attack, but The Flash was saved by Doctor Fate just before Barry would have been killed. In The Flash's absence, Red Death infected Central City with a storm made of Speed Force energy, enabling him to rapidly accelerate the aging in others up to their death. Red Death made an attempt on Barry's Allens loved ones with this ability but was stopped by Barry Allen when he returned.
Batcave or Batgrave?
As The Justice League were onto a solution that would enable them to defeat the Dark Knights, the villains managed to trap them all in Batcaves specifically designed to hamper the abilities of the hero trapped inside. Red Death, naturally, trapped The Flash in a Batcave designed to dampen Barry Allen's powers. Red Death had also created a variety of different Flashmobiles with the intention of killing The Flash inside the trapped Batcave. Despite all of this, The Flash managed to elude Red Death yet again.
The Return of -52 Barry Allen
While the Dark Knights' were enacting their plan to infect the World Forge with darkness, The Red Death was exposed to a burst of positive energy. This positive energy had a two-fold effect; it changed his appearance, but, more importantly, gave control of the body to -52 Barry Allen. This Barry Allen was utterly crushed by what -52 Batman/Red Death had done with the powers they'd stolen from Barry, and did what he could to help The Flash stop the Dark Knights.
Betrayed
Red Death was in a group of Batmen, and if we know anything about Batmen, it's that they do their best to prepare for any eventuality. They'd taken into account the dual minds trapped in Red Death's body, and concluded that they'd need to keep an eye on him to ensure they wouldn't be betrayed. Fortunately for them and unfortunately for both Barry Allen's, The Batman Who Laughs reveals that the positive energy that infected Red Death would soon kill him. Moments later, -52 Barry Allen explodes, taking -52 Batman and, in turn, Red Death, with him.
RED DEATH IN THE CW?
It's certainly possible. In an earlier episode of this season, when Barry caught a glimpse of the future Flash Museum, he caught a recording about one of the main antagonists from this season, Cicada. In the recording, the speaker mentions that Cicada killed more people than "even The Red Death". So, is Red Death coming in a future season(s) of The Flash? Maybe. If so, his backstory would likely be altered in a major way, likely not including the Dark Knights whatsoever. But, instead of Batman being the one merging himself with The Flash, what about an alternate version of Oliver Queen?