It has been a long time since I've published on this site. Life has been a little bumpy and so haven't had time or the will really to want to write stories anymore. Recently started watching Supergirl as well as getting into the rest of the arrowverse in general. Also Kamen Rider Build, one of the best Kamen Rider series in recent times is coming to an end so I decided to explore this story idea of having a crossover between Build and Supergirl. Hope you all like this. Also in regards to all my other fans, I have deleted some of my other stories that I will not be willing to dedicate effort to. Those still left online are still up for debate as I want to see this idea to completion first. If anyone would like to adopt one of my stories, feel free to hit me with a PM.

Leo Titus muttered a curse as he nearly slipped from a step in the abandoned storage facility he was currently exploring. The facility was located some ways outside of National City and one could get a view of Fort Ross from here. Although to the public the facility was once a simple warehouse operated by the government, it was in actual fact once the headquarters of the Department of Extra Normal Operations. It was also the place that once held some of the escaped captives of Fort Ross, an alien prison that had crashed landed on earth a few years ago.

The 23-year-old astrophysicist was working in his lab at the DEO when one of his instruments displayed some impossible reading that happened to coincide with the extremely rare planetary alignment phenomenon. Without telling anyone in the DEO, the brown haired young man had packed his bag and instruments and slipped out of his lab, changing into a DEO tactical suit once he arrived at the location. He had been combing the area for the past five hours, navigating the poorly maintained building and narrowly avoiding several accidents along the way.

'The readings seem to come from here. The tunnel where the elevator to the lowest levels of the facility where the prisoners were housed once was. I guess I'll need to go down and take a look.' Leo thought to himself as he set his bag down and proceeded to unpack some gear to scale the tunnel.

He had tied the rope securely around a nearby pillar and after checking that everything was firmly secured including his harness. He attached the rope and started scaling down the tunnel, unable to see anything below him. His instruments were all going crazy from the readings, suggesting to him that he was on the right path. However, the rope that he was using suddenly snapped and he found himself plummeting down the dark tunnel. He closed his eyes as he prepared for the inevitable impact that was waiting for him once he reaching the end but was surprised when after what seemed to be ages, he felt like he was floating. He opened his eyes and was surprised when he found himself surrounded by some sort of clear liquid.

'Where am I? How am I breathing in this liquid?' Leo thought to himself as he looked around. He was surprised to see a large glowing cube in front of him. He suspected that he was looking at what caused the readings on his instruments and out of curiosity, went to touch the cube. He was engulfed in a red glow before he found that his surroundings had changed once again.

'This is just getting plain freaky. First, I'm suspended in liquid. Now it looks like I'm in some sort of universe where Armageddon happened.' Leo thought as he took in the surroundings of a decimated National City, recognizing several landmarks that were nothing more than piles of rubble now. He chose to focus on the impossibility of the situation to prevent the fear from gripping him.

"This is what will happen to your world if they are not stopped." A voice spoke from behind Leo. Leo turned around and saw a lady dressed in white robes holding the glowing cube in her hands.

"Who is this 'they' you are referring to? And is this some sort of possible future?" Leo asked.

"This is humanity's current future. They are known as worldkillers. Similar to the champion you call Superman, these three kryptonians, known by the names of reign, pestilence and purity will bring ruination upon your world." The woman said solemnly.

"Isn't there any way to stop them? I mean not even superman is completely invincible. We could just shoot them with kryptonite or something right? Use red sun radiation?" Leo asked.

"Though they are the same species, their source of power is not the same. They draw their power from dark magic, growing stronger as they engulf the world in darkness." The woman said.

"So how do we stop them? If they have the same strengths as superman but none of the weaknesses then how will we stop them?" Leo questioned.

"There will come a time when humanity greatest defenders will make a stand against them but even they will not be enough. Not without sacrifice or without the powers of this cube. This cube is a wellspring of nebula energy. It was sent here in the hopes of finding the right person to harness its energy in order to prevent such events from coming to pass." The woman replied as she held out the cube to Leo.

"Wait but why choose me? Wouldn't it be better if it was in someone else's hands? Like Superman's for instance?" Leo asked.

"The cube's energies cannot be harnessed by someone already possessing powers. It has already chosen you Leo. It has spoken out to you. In time, you will harness its powers, surpassing your human limits and gaining power unlike any seen before." The woman said as she took Leo's hand and placed the cube in his palm.

"Well I guess I don't really have any other choice now do I?" Leo admitted as the cube glowed brighter.

"Farewell Leo Titus. One last piece of advice, find your center. It will keep you anchored through the change to fully harness the power of the cube." The woman said before the glow overwhelmed Leo and he had to close his eyes. When he opened them again, he was lying on the floor next to the tunnel. He opened his palm and was surprised to find the cube inside.

'I think I'll study you in private for now. No need to push too much attention on myself.' Leo thought before placing the cube in a container and shoving it into his pack. He cleared up the rest of his equipment and decided to head home after calling the DEO to start on his research on the cube.

And that's the prologue. Basically my OC, Leo discovers this Universe's version of Pandora's box together with information of what is can be used for. He also receives a hint of the threat to come. This chapter takes place around 3 years before the start of canon Supergirl. Next chapter will be straight to canon. A few things for my readers to help me decide:

*Spoiler Alert*

For those of you already familiar with Build, I do plan to have him get upgrades throughout the series. There are however some of the upgrades that I'm not certain to include:

1) Cross Z Magma (As an alternative to RabbitRabbit and TankTank)

2) Genius vs Evol Black Hole

I do plan on featuring the Evol Driver as Leo's final upgrade and that dovetails into the Black Hole Form however if enough readers feel it doesn't make sense for a protagonist to use it then I may be tempted to introduce Genius instead.