Fusion energy.
Doom Slayer had gifted Earth the capacity of building a fusion reactor, and incidentally broke the tendency of weaponizing gifts. He essentially put the power of the Sun on their hands. And all because, if he was being sincere with them, they had 'reached the level', a phrase that few, if anything, knew the meaning of his words. Some people, however, did understand what Slayer said. Earth had had the technology to create fusion reactors for several years, but the men or companies that had that knowledge didn't share it with the rest of the world for several reasons, all of them radiating from the general belief of humanity not being ready to use such power, given that the only real use they had until that day was in the creation of thermonuclear weapons. Add in a disaster like Chernobyl, and it was easy to understand their reluctance.
Doom Slayer's actions, and him being Doom Slayer, proved that they could use it.
As for how a fusion reactor worked, it's simple: the design provided several times more power than a regular fission reactor, and required deuterium to work rather than uranium, but it wasn't that much bigger, making it more efficient.
Maybe a coincidence, maybe out of symbolism, Slayer asked the ones in charge of the project to build the device and the power plant it would be housed in New Mexico (others would be built on those countries able to afford one, but that one would be the first one and testbed for the others).
Naturally not everyone in the world saw his actions approvingly, and not just those who still believed man could not be trusted with such power, ranging from his usual opposers, to energy magnates who saw the profit their companies made drop dramatically in the future, to simply people who did not want something so unsightly in their homes.
But as always, Doom Slayer ignored them and their pretentiousness, while assuring those whose concern was genuine. In the end, however, the history of Earth had changed forever.
And it would only keep changing.
To be honest, I expected more people around the world to react to the fact that I just gave them something some had only thought of in their wildest dreams, but in the end the excitement and amazement died out, which given the kind of world this Earth is made sense. Meanwhile, I helped build the first reactor in New Mexico.
After that it was time to return to the search for Rachel. At that moment we only had the Green, the Blue and the Red, but John's kidnapping and Tala's intrusion, to put it mildly, threw a wrench into our plans. No longer could we just go for the other links to the Elemental Realms knowing that a powerful magic user was out there.
It was time for John to do his part of the mission... but with only three links, he couldn't do it himself, no matter his experience: his magical pool was simply not big enough for it.
Fortunately, Bruce knew of someone who turned out to be lacking in skill, but an order of magnitude higher than the vast majority of witches, sorcerers and mages out there. All we needed from her was a bit of her power, and John would do the rest.
Sadly, she also realized what I was the moment she laid her eyes on me.
Who'd ever thought that the stage magician Zatanna Zatara turned out to be an actual witch? Or was it mage? Anyway, she could do magic, real magic! Not that she didn't love doing her usual tricks, but pulling a rabbit out of a hat without a top had to be the oldest, most boring 'magic' trick in the world, one that she got tired of performing (not so her audience).
But pulling a tiger out of a normal hat? One that was supposed to be riding a unicicle in Las Vegas? Granted, the entire thing ended with her covered in claw marks, but still! And all that from speaking backwards!
Then, one day, her father called her because he had detected her awakening from afar... and did so after using some magic portal thingy to go to see her face to face. It turned out that her father had been a true wizard himself, but had to abandon his path for several reasons, almost none he told her of aside from meeting her mother.
And then several days after THAT, out of the blue. Bruce told her he was Batman, THE Batman! The man that she had had a crush on before leaving it on just friends was also the Dark Knight! And instead of just keeping the truth to himself like he had done for years, he just revealed it to her! That... alright, she didn't really understand why did he do that.
A couple of months after that, Bruce told her to meet him on his manor, telling her that it was an urgent manner. The fact that Bruce of all people said it was urgent was enough for her to cancel the show of the day and race to Wayne Manor, still wearing her fishnets and leotard. What could be pressing enough that Batman required her help, she didn't know, but was more than happy to help him out.
She met Bruce, who was already awaiting for her on the gate of the manor, and after some small talk and asking about his call and receiving a vague response about 'guests', followed him inside, where she saw firsthand the previously mentioned guests.
One was a raggedy blonde man who whore a leather jacket and blue jeans, smoking a cigarette and smelling both of tobacco and alcohol even at that distance, and who at that moment was exchanging insults with Alfred about Shakespeare and Glasgow, and not in a good way going by Alfred's annoyed expression. Judging by his accent, he was a fellow englishman, but the kind that spent their lives screaming at TV and throwing beer bottles at it.
"And I'm telling you, Mr. Constantine, that James the Fourth can not be a descendant of Macbeth. And could you please smoke outside?"
"My arse he wasn't! It's too damn obvious to be otherwise! And last time I came you didn't care!"
"Because last time you didn't smoke at all."
"Don't deviate from my question you ponce!"
Zatanna had to stop herself from reeling, even though the guy could be considered handsome with that rugged face and blonde eyes of his. He seemed to be the kind of man who wanted to deliberately make himself loathed by everyone just by existing.
But when she laid her eyes on the other guest... she froze.
After all, she never expected Doom Slayer of all people to be there.
Technically she had helped him before by giving Bruce a magic detector from her father to help them on their search (she didn't tell anyone she didn't make it), but again, only through Bruce.
Thing was, it wasn't just his sheer size and unmoving posture what freaked her out. She couldn't really see what exactly was coming from Slayer, but she knew what it was: power, and not in the sense of being super strong (which admittedly he was), but in the sense of emanating mystical energy like some sort of magical nuclear reactor, a powerful reactor. And she still didn't understand much of real magic.
"Excuse me, I need to, er, take a breather." She said before walking out of the living room. Then, she leaned on a wall. "Damn."
"Zatanna?" She heard the deep voice of Bruce ask. She didn't bother to question how didn't she heard him. "Are you alright?"
"Don't worry, I just needed some air."
Naturally, Bruce wasn't so sure. "Are you sure?"
"You're showing concern now, Bruce?" She chuckled. "I'm sure, it's just... Doom Slayer, here, in your house!"
"Are you afraid of him?"
Zatanna snorted. "Of course not." But Bruce kept frowning at her. "Alright, but not because he'd kill me or anything, I'm past that. I'm barely five point five, and he's two feet taller than me and built like a tank, and that's without taking into account what he did last week. And now I'm meeting him in person. Me, some magician from Gotham. I think I have a right to be overwhelmed. So, Doom Slayer wants my help, doesn't he?"
"Pretty much, but it's more like he needs your assistance."
"Is there a difference?"
"You might not be able to help them directly."
Zatanna lifted an eyebrow in confusion. "Then why does he need my help?"
"It'd be easier if you asked him yourself. Believe me, it's complicated, even for me."
Confused, Zatanna entered the room again, and this time the raggedy man was also sitting, if still giving a side glance to Alfred. Doom Slayer
The giant man approached her, and Zatanna noted with some trepidation that he seemed even bigger up close. And then he spoke. Hearing him speak talk in person and not through a screen was a sobering experience. That said, aside from being obviously deep and baritone, his voice had nothing else special about it. Maybe that's how he somehow being special in everything else?
"Miss Zatara, this" He said as he gestured to his companion. ", is John Constantine, my confidant and the one who will tell us if you can aid us."
The man, now named Constantine, stood up and got close to Zatanna, and began inspecting her with his sight... and nose. At least he didn't touch her. "Yep, she's got it, but still smells like cheap perfume. And also you look like some corner tart with those things on, did you know that?"
Understandably, Zatanna didn't like his comment. "Says the one who looks like drunk Columbo. And Bruce called me right in the middle of an act, didn't have time to wear someting more covering."
"Drunk, ghost-fighting, demon-banishing, dogless and british Columbo, thank you very much." Constantine replied, ignoring her last sentence. "Well, technically I have a dog, but he's also a kid, and he's yet to actually turn into a dog." He then turned towards Slayer. "Has he?"
"He can turn into an african painted dog, so yes."
"Wait, you mean you're a father?" Zatanna said, surprised by the fact such a jerk could be a parent. Only...
"Of course not, he is." Constantine said as he pointed at Slayer. "I'm more of that uncle who's forced to smoke out of the main hall because the overbearing daddy doesn't want little Tarzan boy to grow with the smell of tobacco, and because the houseguests don't like it either I have to do it on a dank, dark cave underneath the surface, where I am the one risking asphyxiation. And the only time he allows me to take a whif is after I nearly got killed by Baldy." As calmly as he said it, it was obvious the englishman was ranting.
Zatanna, however, was too busy looking at the future man with wide eyes. Doom Slayer, the same Doom Slayer who had proved himself to be more than willing to kill his enemies in a brutal manner, was a parent? And was raising the kid on his Fortress of all places alongside Solomon Grundy (and a chainsmoker and drinker too)? When did that happen? Hell, how did that happen?
"You're lucky I don't force you to the surface proper." Slayer said as he crossed his arms over his chest. "The couple doesn't like it either, remember?"
Zatanna merely looked at Bruce, who in spite of himself actually rubbed the back of his head. "It's a long story."
"Alright." Constantine said as he clasped his hands and looked at Zatanna. "Let's stop fucking around before I get riled up. First, your spellcasting. I've been told you pulled a tiger, a living breathing one, out of your hat. How did you do it?"
"By spelling words backwards."
Constantine raised an eyebrow at her response, clearly confused. "Really, just like that? Not absentminded gestures or anything else, just subliminal messages?"
"I've always said phrases or words backwards when performing my magic tricks. Never knew I was the real deal." Zatanna replied in a defensive tone. "And how do you do magic anyway?"
"By doing hand gestures and saying stuff in latin. At least that's how I do it. Anyway, give us a private show, twist our brains or something like that. I need to see it with my own eyes to gauge you."
"Don't tempt me."
Zatanna began to think about, well, what trick she could do with actual magic, and naturally found none. And then she remembered, and a thought came to her mind, one that made her smile a mischievous smile. Incidentally, that same smile made the englishman frown.
"You better not think about burning my hair or something like that. Last one who tried ended up bald."
Zatanna didn't bother to give him a straight response. "Ecin llems nam yllems ekam!"
At first nothing happened, but as Constantine opened his mouth to talk, he then caught a whiff out of himself. "Well, colour me impressed, it worked, can't smell the scent of nicotine and alcohol. Could have done something more impressive, but it worked."
"I give it a day, maybe two before your stench comes back." Alfred deadpanned.
"Maybe I take you to sleep with Grundy instead, twat. I bet you wouldn't last-" The wizard managed to get out before Bruce interrupted him.
"John, stop it, please, and leave Alfred out of your spats."
Constantine threw his arms in the air before turning to Zatanna. "Fine, fine, I'll stop it. Anyway, this proves it: she's the one that we seek."
"Before you go on" Zatanna began to say, arms crossed. ", could you guys tell me a bit more about, well, everything? Because I'm pretty sure you haven't told me anything yet. All I know is that Bruce called me to come here and, well, that's it."
"First things first then. You radiate mana like one of those expensive batteries from Germany, which in this case is a good thing."
"So, you're saying I'm like a living reactor of magic." She said, trying her damnest to not look at Slayer.
"Pretty much, yeah. With some practice you could be pretty much the most powerful magic user on Earth, or at least among those, but that's not why we called you here. Remember the magical counter thingy you gave to Bats here, and he to us?" He said, earning a nod from her. "Did he tell you what did we need it for?"
"Of course, to help you in your search for Solomon Grundy."
"Yes, but did Bruce tell you why did we want to find Grundy?
"To try and make him join you? Because that's what he did."
Constantine and Doom Slayer exchanged a glance. "Not exactly. It was more of a secondary objective, really." Slayer replied.
And so Constantine and Doom Slayer told her about his and Doom Slayer's goal, about a plot that covered over a hundred years, about ancient magic linked with the forces of nature, and the forces of Hell doing their damnest to cross into Earth.
"You're saying that this... Arella woman linked several people in the past to make a spell that would hide her daughter's essence to hide her from her demon father?"
"There are more of them out there, but we're running out of time, and I can't do much without them and the power they hold."
"And the reason you need me even though I'm not experienced with magic at all is...?" She inquired.
"We want to use you as a battery for me for a spell that may or may not kill you if done carelessly." Constantine responded. Seeing her bewildered expression, he shrugged. "You asked."
Well, at least he was being honest.
The story's back from the hiatus I think I rushed the 'nuclear fusion reactor', but let's be honest, that plot bunny had to leave. Nonetheless the SI giving the knowledge for the world to make theirs does have an impact, it's just not important to the story (at least for now).