Based on a Tumblr Post by Supercanaries: post/151156574056/supergirl-dcs-legends-of-tomorrow-au-in-the

In the year 2017, Kara Zor-El is lured to save her sister from an evil secret organisation. The kidnapping is a ploy to unleash red kryptonite on Kara and have her destroy National City. However, when Kara accidentally kills Alex, the darkness inside her grows exponentially until she destroys all Earths. Rip Hunter sets a course to the future of Kara's Earth and sends Sara to deal with Supergirl, both because she understands being devoured by darkness and because she relates to losing a big sister. Instead of letting Alex die, though, Sara decides to go save her in Kara's place.

Chapter 1

"What's the emergency Rip?" Asked Sara as she entered the room. She had no idea why Rip had summoned her in here, only her, or what he wanted but he did not look happy. Rory was stood beside him, looking annoyed as usual. "You got a solo job." He said, "Some alien chick destroying stuff."

"Wait, what?" Asked Sara, now extremely confused.

"Gideon, show her." Ordered Rip.

The image of an undeniably beautiful woman in a black jumpsuit appeared on the screen and Sara fought the urge to flinch. She was laughing in a way that made Sara afraid. An angry, hate-filled, yet somehow gleeful laugh. Then there were gunshots and...oh crap those bullets just shattered on her. Her features solidified into, what could only be described as pure rage, and she lunged out of frame.

"Meet Kara Zor-El, known on her Earth Kara Danvers." Explained Rip, "A Kryptonian who was sent away as her planet was destroyed. She ended up a hero named Supergirl, but in 2017, an organisation kidnapped her foster sister, Alex. While trying to save her, she was exposed to a substance named Red Kryptonite, which massively enhanced her rage. As she attempted to destroy the base, she accidentally destroyed a support beam and Alex was crushed. When she realised her sister was dead, the darkness in her grew, to the point where she destroyed, not just her Earth, but all Earths." Rip commanded Gideon to shut down the image as cries of agony filled the room. "Your mission is to stop Miss Danvers from trying to save her sister. If she does, Alex will die and Kara will destroy everything."

"Aliens," Scoffed Rory, rolling his eyes, "I hate aliens."

Sara considered everything she's just been told. Rip wanted her to stop an alien. A pretty, seemingly invulnerable alien who was capable of destroying everything.

"Why me?" She asked, "She could probably kill me in 5 seconds flat; why send me?"

"Because you understand." Said Rip, gently, "You know how it feels to have the darkness consume you and, more importantly, you know how it feels to lose a sister." Sara froze before slowly backing towards the door.

"Are you saying that you want me to go to this girl's Earth, let her sister die and deal with the fallout?"

"Not at all Miss Lance. I'm saying, you stop HER from doing the saving." Rip sighed and walked over to the assassin, placing his hands on her shoulders, "There is no-one I would trust more with this one Sara." Sara stared at him for a moment, before giving up and strapping herself in.

This is not gonna go well.

D.E.O Headquarters

Kara was pacing in the center of the HQ. J'onn was honestly starting to worry about her wearing a hole into the floor. To Kara, however, it didn't matter; nothing mattered because Alex wasn't here. It should have been a simple mission, how did she let that happen?! They'd taken the bad guy out (with Kara commenting that he "took more punching than usual") and Alex had escorted him into a D.E.O van while Kara went to deal with a fire in the centre of the city. It was less than ten minutes later that J'onn had called her and told her the van was abandoned, the other agents were dead and Alex had disappeared.

"Supergirl." Kara froze and her glare fixated on one of the tech guys. He visibly shrunk under her gaze, clearly worried about being vaporized. "I-I received an e-mail. It says they'll reveal Agent Danvers' location tomorrow, along with instructions."

"That's all you've got!" She took two steps towards him before J'onn grabbed her arms and dragged her out of the room.

"Kara, there is nothing we can do. The whole organisation is looking for her, but the best thing you can do right now is go home. Go to CatCo, do work and leave our guys to do their jobs."

"But..." Kara tried to protest but the Martian just held his hand up.

"If the e-mail is telling the truth, we won't get any exact information until tomorrow and you still need to keep up appearances." He placed his hands on her shoulders gently, "We'll find her Kara, I promise." Kara sighed and angrily wiped the tears from her eyes.

Kara left the HQ and headed towards CatCo. Work may have been the last thing on her mind, but J'onn was right, she wasn't much use until they knew Alex's exact location, so she figured overtime planning the press party was as good as anything else. The closer she came to the building, however, the more tears started to prick her eyes and the more hopeless she felt. Eventually, she changed direction heading to her apartment as fast as she could without using super speed. Closing and locking the door, Kara ran into her bedroom, collapsed onto the bed and began to sob. She needed Alex. How could she have let it happen?

Cheerful music rang out within the room, causing her to look up. Her phone, carelessly discarded on the bedside table, was ringing obnoxiously loudly. With a groan, she reached over and grabbed it, wiped her eyes and answered it. "Kiera, is everything ready for the party tomorrow?" Kara sat up immediately, sniffing slightly before replying, "Um, almost Ms Grant, I have to call the DJ first thing to confirm the set list, but everything else is ready."

"Excellent, I expect to see you there, no excuses." Before she could reply, Cat hung up and Kara fell back onto the bed, running a hand over her face.

Things cannot get any worse...