DISCLAIMER: I do not own Supergirl or Quasar, neither do I own DC or Marvel Comics characters or characters from the CW Arrowverse.

Author's Note (AN): This is a crossover between the CW Supergirl show, DC comics, and Marvel Comics.

Prologue: The End and the Beginning

Titan

The final battle against Thanos was long and hard fought. Debris was strewn around the ruin of the capital city of Titan. The planet was quiet except for the unsteady breathing coming from a pile of debris. A man in a tattered red, white, and blue set of scaled armor sat up against the pile of debris. His mask had been ripped from his head; two large gashes were visible along the side of his face. His left leg was clearly broken and he breathed unevenly, signaling that he had a punctured lung amongst a multitude of other injuries. Two people knelt on either side of him; a woman with short blond hair wearing a skin tight uniform that was red and blue with the gold emblem of a star on her chest. On the other side was a younger man, most likely in his early to mid twenties; he had blond hair that reached his shoulders. He had a handsome but very youthful looking face with a dimple on his chin and blue eyes the color of glacial ice. His eyes were filled with unshed tears as he looked down at the man.

"Cap, hold on, you're going to make it," the young man said.

Cap forced a smile but the grimace that took over his features revealed the amount of pain that he really felt. "It's too late for me, kid," he replied softly.

The woman shook her head. "We can still win this, don't give up."

Cap shook his head twice. "No dice, Carol; you're both the last of the Avengers. You need to do what you can to defeat him. To save this Universe."

Carol had tears in her eyes; she held onto the man's hand. "We'll avenge you, Cap."

Cap smiled. "Avenge all of us."

The younger man nodded. "I promise." Cap nodded once before his eyes glazed over and his shallow breathing stopped. Carol looked away, but the young man leaned forward, closing the eyes of the Sentinel of Liberty. The young man stood up, his blue, red and black uniform fully revealed as he stood to his full height of six feet. A gold emblem like a four pointed star was on his chest and his blue cape held a black background on the inside with stars that pulsed with energy.

Carol sighed. "What did I tell you Wendy, about promising?"

"I hate that stupid nickname; the name's Wendell Elvis Vaughn," he replied sarcastically.

Carol smiled mischievously. "Wow, you're mother must have been high when she named you that."

Wendel stood before Carol with his hands on his hips. "You can make fun of me all you want Captain, if we live through this."

Carol frowned. "That's just the thing," she said before a golden aura of power covered her body before she flew off at full speed.

"Fuck me," Wendell said before the large wrist bands on his forearms began to glow, covering his body in energy. He floated into the air, taking off at unimaginable speed to catch up to the woman known as Captain Marvel.

He reached an abandoned clearing, the quantum bands on his wrists flashing, warning of the danger ahead. He stopped in mid-air, floating about twenty feet in the air, his eyes wide at the sight before him. Carol Danvers holding onto the restored Infinity Gauntlet as the mad Titan, Thanos poured all his power into the woman that dared to hold the gauntlet. Carol's body absorbed the energy, taking everything the gems could muster as her body turned into a small sun. Her skin turned red and her hair turned to flame. She was Binary now. The one form of Captain Marvel that could hope to withstand the power off all six Infinity Gems.

"Carol, no," Wendell yelled. Carol smiled sadly at her young friend, mouthing the word "run".

Wendell froze for a moment; run, that's what she said. How could he run and leave her, but he finally realized what was going on as his Quantum Bands registered that Carol's Binary form was about to explode with the force of a supernova. He closed his eyes before powering up his Quantum Bands, initiating a quantum jump just as his friend exploded, ending her life and that of the mad Titan.

The Alien Earth - Home to Kara Zor-El, also known as Supergirl

Kara stood on her balcony, it was a week since the final battle with Reign and the Kryptonian witches. The Legion had left and Mon-El went with them, his presence altering the timeline too much for him to stay. They both cried, but in the end Mon-El's duty won out and she could not fault him for that. She had continued her sabbatical from work and contemplated going back with her mother to Argo City, but kara had decided to stay. Argo City wasn't her home, Earth was.

Kara's phone rang, breaking her away from her sad thoughts. She answered the phone when she realized it was from Alex. "Alex," she said.

"Oh, thank God, we have a situation here. A meteorite of some unknown type is heading toward the outskirts of National City. Can you get there and make sure no one is hurt? It will take my team several minutes to get there."

"I'm on it, just send me the coordinates," Kara replied before Alex hung up the phone, sending her the map coordinates a second later. Kara ran full speed into her room, changing into Supergirl before flying out her window and into the sky. She poured on the speed racing toward the meteor that she could now see in the darkened evening sky. It looked like it was headed to an unpopulated area. Kara focused on the meteor, using her enhanced vision to see into the meteor's interior. Her blue eyes widened in shock when she saw the outline of a humanoid body. She poured on the speed but her slight hesitation was enough to allow the meteor to strike the desert landscape, throwing up clay soil and knocking over a few cactus.

She hovered over the crater created by the meteorite impact; it was unlike any meteorite impact that she was familiar with, it made a small impact and the heat from the meteor was non-existent. She moved toward the source of the impact, landing on the ruptured surface of the desert. She stopped in her tracks at the sight before her; a man was sprawled out in the crater his body still had some steam coming off it. She walked carefully toward the unknown man, her mind flashing back to a previous crater that held a Kryptonian pod and a certain Daxamite prince. This time there wasn't a pod, only the body of a young man. Kara knelt down beside the man; his face was bruised and his blond hair was singed slightly, but all in all, he seemed perfectly fine. She reached out and touched golden symbol in the man's chest. The man groaned; Kara removed her hand in an instant.

Wendell Vaughn, also known as Quasar opened his blue eyes, looking up into the equally blue eyes of a young blond woman. She wore a blue and red uniform with a red "S" on her chest. He closed his eyes a moment later as the exhaustion finally took over.

The woman smiled, blushing slightly at the handsome features of the man that were clearly visible through the bruising and lacerations. Kara forced those thoughts away; she couldn't project her emotions onto someone else now, she needed to be strong and force away her attraction to this unknown person. As she was internally musing she hadn't even heard Alex and her team arrive.

Alex nudged her sister's shoulder, startling Kara. "Wow, you must really be lost in thought," Alex replied sarcastically. Kara glared at her, but Alex only smiled in return. "So, do we know anything about our mystery man here?"

Kara shook her head. "I only just arrived before you. I can sense that he's still alive, but that's all."

"Did you x-ray vision him?" Alex questioned.

Kara blushed, not wanting to admit that she totally forgot about doing that. She quickly scanned the man. "He appears human."

Alex frowned. "No human could have survived that impact."

Kara nodded. "You're right, maybe he's a metahuman."

Alex sighed. "Well, we'll need to take him to the DEO, place him in the infirmary under constant surveillance."

Kara frowned. "We don't know he's a threat."

"We don't know he isn't. I'm not taking any changes."

Kara relented. "I'll be around later to check on him," she said before taking off into the air.

Alex sighed, looking up into the air as her sister flew off. She knew that Kara was still hurting because of Mon-el leaving. She understood why he left, but she'd always be angry with him for leaving her sister. She realized that Mon-el had never been right for her sister. She hoped someday and somewhere there was someone out there for her sister that would make her happy, truly happy. She looked down at the crater again as she watched her team place the man on a stretcher and carry him away to a DEO medical van.

"Now, back to you, Mister. Who and what are you?" she asked out loud to no one. She walked away, entering a black SUV before driving back to the DEO Headquarters.

DEO Headquarters - Infirmary

Wendell Elvis Vaughn slept comfortably in one the DEO's medical beds; however, his comfort quickly subsided as a nightmare of a very recent encounter played over in his mind; the deaths of his friends, of the Avengers. He relived the death is his mentor, Captain America and of the woman he considered an older sister, Captain Marvel.

He suddenly jumped up in bed looking around at his surroundings, realizing that he was in a medical wing of some office or base. He hoped it was SHIELD or SWORD, someplace he was familiar with, but no, he had never seen this place before.

Suddenly, the door to the infirmary opened and a young woman entered, dressed in a blue uniform with a red cape and a red "S" on her chest. Wendell thought that she was a very beautiful woman, but he forced away his feeling, this was no time to act like a fool teenager; he needed answers. "Where am I?"

The woman stopped walking, realizing that the man was awake. She froze at the sight of his glacial blue eyes; they were beautiful. "This is the DEO Headquarters, in National City."

Wendell's eyes widened. "Where?"

Kara thought the look of confusion on his face was cute. "The Department of Extranormal Operations, in National City, California."

Wendell shook his head. "There is no National City, California, and I never heard of the DEO." He tried to sit up more but his arms were bound to the bed. "Why am I tied up?"

Kara looked at his bound arms, cursing Alex and J'onn under her breath for making the wrong first impression here. "I'm sorry about that, but my sister and her boss get a little scared when someone falls from the sky like a meteor striking the Earth."

"Meteor?" Wendell asked, looking up from his bound arms.

"Yes, you landed on this planet, or crashed to be more accurate."

Wendell shook his head. The quantum bands powering up, breaking through his bonds effortlessly. Kara stepped back at the sight. Wendell moved, jumping from the bed, his feet hitting the floor, making him stumble slightly. Kara moved to help him, but Wendell recovered quickly, placing an energy shield between himself and Kara. "Stay back," he said with a voice of a leader.

Kara touched the energy shield, trying to push through with her hand, but the shield was incredibly strong. She doubted that she could punch her way through it.

Alex and J'onn entered the room. Alex pulled her weapon, aiming at the now fully awake man that she brought to the DEO headquarters earlier that day.

J'onn raised his hand. "Agent Danvers, lower the weapon." Alex holstered her gun, but kept her hand on it just in case. J'onn turned back to the mystery man. "My name is J'onn J'onzz, director of the DEO. Who are you?"

Wendell thought about giving them his real name, but he thought better of it quickly. "Quasar," he simply replied.

J'onn and the others studied the young man closely. "Nice to meet you, Quasar. Can you tell us why you are here?"

Quasar studied the three people in front of him and realized that he really didn't feel that they had any negative intentions, in fact, it appeared that they wanted to help him. He decided he was going to go against the old comic book cliche fight with other heroes. He lowered his shield causing the Kara and the other's to look at him curiously. "Sorry about the shield; I don't like to be tied up."

This man became more curious to Kara by the moment. "Why lower your shield?"

Wendell smiled. "Well, I don't sense that the three of you have any hostile intentions toward me, even with the gun toter over there," he said sarcastically, sending an amused glare toward Alex.

Alex smiled. "An old habit," she replied.

Wendell nodded; this lady reminded him of Black Widow. "I've never heard of the DEO or National City. I've heard of California though."

J'onn nodded. "What's the last thing you remember?"

Wendell focused but it was very easy to remember, it was the nightmare that wouldn't go away. "I was on the planet Titan," he said, noticing the blond flinch at the name. "My team and I were fighting the mad Titan, Thanos. He had a weapon called the Infinity Gauntlet and he wanted to destroy the Universe. We fought him until only two of us were left."

"What was your team called?" Kara asked curiously.

Quasar looked at the blond. "We were known as the Avengers, Earth's Mightiest Heroes."

Kara smiled. She thought the name was interesting, but not as much as the young man before her.

Alex smirked at her sister before her gaze returned to Quasar. "You said that it was just you and one other."

Quasar nodded. "Carol Danvers, also known as Captain Marvel; she was like a big sister to me." Alex and Kara both looked at each other at the name Danvers. "She sacrificed herself so that I could live."

J'onn nodded; he didn't have to use his psychic powers to realize how much pain Quasar was in at the thought of his friends. "Any idea how you got here?"

Quasar nodded. "When Carol used the last of her power to defeat Thanos, I used my quantum bands," he said raising his arms into the air to show the bands that covered his wrists and part of his forearms. "I can used them to fold space and time to move great distances, but something happened... an explosion... after that I don't remember anything except waking up here."

"Could this explosion explain why you ended up here?" Alex asked.

Wendell nodded. "This must be another Universe. It's happened to me before but not in this way."

Kara smiled. "I've been to other Universes too."

Quasar smiled; he couldtn't help it, her smile was infectious. "So, I've been introduced to J'onn," he gestured toward the DEO director. "I haven't been introduced to the two ladies here."

J'onn smiled. "Ladies," he said.

Alex nodded. "My name is Agent Alex Danvers, second in command of the DEO."

Kara stepped forward. "My name is Kara Zor-el, I'm not from this Earth. I'm Kryptonian actually and I go by the hero name, Supergirl." she reached out her hand before Quasar extended his hand as well. When they clasped hands a bright light shot forth from the quantum bands, flooding the room with light. When the light faded Alex and J'onn looked around but Quasar and Supergirl were both gone.

AN: That's the end of the epilogue. Please favorite and send me a review to let me know if you want me to continue with this story.