Mar-Novu, a cosmic entity, who's background/origin is shaded in mystery, is the culmination of the positive energy of the multiverse, whereas his brother Mobius, is the creation/result of the negative matter.
"In the beginning, there was only one… a single black infinitude. Then the infinitude found release, and finally, the darkness broke, filling it with life, with the multiverse. Every existence multiplied by possibility and spread out before space and time in infinite measure. Civilizations rose and fell and rose again to cross realities grasping expanse. Life, a precious gift persevering in the face of every obstacle, until finally, the age of heroes was born. Chaos, the constant enemy of life, kept at bay by champions from across the multiverse. Joining forces to fight on behalf of all creation, they found each other just in time." Mar Novu explained, but just who was he explaining this all to because not everyone is equipped to handle such vast knowledge.
Previously on Arrow and The Flash
Earth 1 Starling City
"So that's my story, I spent my whole life searching for the impossible, never imagining that I would become the impossible." Barry says.
"Then, why come to me, something tells me you didn't just run 600 miles to say hi to a friend." Oliver implied.
"All my life I wanted just do more, be more, and now I am, and the first chance I get to help someone, I screw up. I was chasing the bad guy, and someone died." Barry said with guilt, a rooftop in Starling City.
"If you really do this, you're going to make mistakes. I've made mistakes, but the good you do will far outweigh the bad." Oliver responded.
"What if Wells is right? What if I'm not a hero? What if I'm just some guy who was struck by lightning?" Barry questioned with doubt.
"I don't think that bolt of lightning struck you, Barry, I think it chose you," Oliver responded.
"I'm just not sure I'm you, Oliver, I don't know if I can be some… vigilante." Barry stated, uncertain.
"You can be better, because you can inspire people, in a way that I never could, watching over your city like a guardian angel, making a difference, saving people, in a flash. Take your own advice, wear a mask" Oliver said as he used his bow to cling on the side of the next building.
"Cool." Said Barry looking at his friend's exit. Then he took off running back in the direction of Central City.
"Cool." Said Oliver as he watched his friend run.
~Later~
"Hey, by the way," Barry, the Scarlet Speedster of Earth 1, remarked.
"Yeah?" Oliver, The Emerald Archer, inquired curiously.
"You were wrong, when you told me I could inspire people, you said you couldn't, but you were wrong, you can inspire. Not as the Arrow that guy's a douche, but as Oliver Queen." Barry responded.
~Soon After~
Nanda Parbat
"There's a legacy here, Oliver. A prophecy rather. Those who survive the blade of Ra's Al Ghul must become Ra's Al Ghul." Serab explained.
"And what if I say no?" Oliver questioned, with firmness in his response.
"I'm afraid that's not an option." Serab taunted, as multiple league of assassin garbed members descended upon the two of them and drew their swords in Oliver's direction.
"Maseo, you don't have to do this!" Oliver screamed.
"Maseo Yomisharo is no more, I am Al-Serab," Serab screamed as he drew his sword. Oliver warded him off, but the number was overwhelming. Serab made his escape while Oliver was fighting through all the chaos.
~Much, Much, much Later~
Earth 3
"We're not gods, Barry. We're just men that were blessed with extraordinary abilities." Jay Garrick, the flash of Earth 3, informed Barry.
Earth 1
"Oliver, I need your help," Barry informed his friend over the phone.
"Alright, I'm on my way," Oliver responded.
~Later~
Central City
Oliver pulls up on his motorcycle.
"Sorry I don't have super speed, Barry." The Emerald Archer spoke, with a hint of annoyance in his voice.
"It's cool, but listen, we have a huge problem, and I can't tell my team until I figure out exactly what it is we're up against. "Barry exclaimed in a panic.
"Barry, Barry, Barry! Relax, I get it. That's why I told Felicity and Dig, that I need to go on a solo mission, and I had to keep it under wraps, cause anyone, and I mean anyone finds out, and the entire mission is compromised." Oliver said, trying to ease his friend's nerves.
"Now, what are we up against?" Oliver asked, regathering his vulgar.
"I'm not entirely sure yet, beasts?" The Scarlet Speedster said confused.
"What do you mean by beasts, Barry? That is far too vogue. You asked for my help, this is what my help looks like. I need answers. If I don't know who…or what we're up against, how the hell do you expect me to strategize and come up with a plan to stop it?" Oliver asked, annoyed.
"Okay, just calm down. It's not like anything we've ever seen before. Doesn't that say a lot? Considering meta-humans, mysticism, zombies, and aliens. Oliver, what we're about to face, looks as human as you and I do." Barry explained.
"Like a man-beast of some sort?" Oliver questioned with curiosity.
"Sorta kinda, something like that. I think they're vampires. Like those stories we knew as kids, about the notorious Count Dracula." Barry explained.
"Vampires, Barry, really?" Oliver questioned in extreme disbelief.
"Really, Oliver, and I don't have the answers, but we both know somebody who might," Barry exclaimed.
"And who might that be again?" Oliver questioned.
"Mari McCabe. Vixen? Detroit? A totem with animal powers? Help you get your son back from Damien Darkh. Anything ring a bell to you?" Barry questioned, as he seen a look of genuine confusion come across his friend's face.