Cross-posted on AO3

Day 20: Betrayal

Diluc swore as he blocked a hit from an Abyss Herald that was swiping at the Traveler's back. The young man sent him an appreciative smile before focusing on the Mitachurl in front of him, breaking its shield with a burst of Geo before switching to Anemo to finish it off.

Diluc had long since learned to not be surprised by his many abilities.

Keeping his attention on the Herald, Diluc could feel exhaustion setting in. They had been fighting for hours now and the city was barely being held together. The enemies just kept coming in what felt like endless waves. Not even Varka's freshly returned envoy wasn't doing much to help. They had already been tired from walking and fighting on their way back and hadn't had enough time to rest properly before the first wave had attacked.

None of them had been prepared.

The attacks had come as a surprise and they were astronomically lucky that nobody had died yet. They had immediately begun evacuation procedures and moved as many citizens as possible to the cathedral. There were underground paths that would let them out not too far from the Stone Gate. They could flee to Liyue from there and give a warning to the Qixing.

If they made it that far.

With a burst of flame, Diluc slammed his Elemental Burst at the Herald and actually managed to knock it off its feet. Encouraged by the progress, he kept slashing at the enemy, hoping to take it out before it could recover. Just as it was beginning to rise again, Aether was by his side, using his Palm Vortex to trigger a Swirl effect.

The combined elements were enough to reduce the monster to golden particles as it disappeared, causing Diluc and Aether to heave matching sighs of relief. While Abyss Mages could be annoying with their shields, Abyss Heralds were practically nightmare fuel. They were large – almost twice Diluc's height – and didn't even bother with shields. Their tough armor (or was it skin) did enough protecting for them to not need them.

If they ever had to fight one of these things again, it would be far too soon.

The sound of clapping caught both of their attention, causing them to tense and look up. What they both saw was enough to make their hearts drop.

Lumine and Kaeya stood side by side on the city walls as they watched the chaos the city had descended into. Lumine's face was completely blank, seemingly uncaring as she watched the city burn to the ground. Kaeya – the source of the clapping – just smiled like he usually did when he wanted to unsettle his opponent, calm and completely unaffected. He looked so much different from the last time the elder saw him, soaked in rain and a newly acquired Vision in hand.

It made something ugly twist in Diluc's chest.

"Lumine," Aether called out, his voice sounding just on the edge of breaking, "what are you doing?!"

The girl turned to look at her twin and Diluc didn't fail to see how she twitched when the siblings made eye contact. She didn't back down, however, and made sure to keep her facial expression void.

"I am merely doing this because it's what needs to be done."

It made Diluc's skin crawl with how clinical she made it sound. Like all this pain and suffering wasn't something that could have been avoided.

"Are you saying that you don't care about the lives you've ruined today?" he asked, his voice slowly rising with his anger. "About the people you've likely scarred for life? The children you've traumatized?!"

He could see she was losing hold of her resolve when she clenched her fists. If Aether's stories about his sister were to be believed, she had been a kind soul who had helped anyone and everyone she could. What could have changed her into this? Made her think that what she was doing was absolutely necessary?

"Whatever the Princess wants," Kaeya started, turning to the shorter girl, "the Princess gets."

He took her hand gently into his own, raising it to his lips as he kissed her knuckles before giving a light squeeze. In that moment, it was like an unknown exchange had happened as Lumine's face hardened in resolve before turning back to her brother.

"This will not be the last time we meet, brother. If you want to oppose the Order, then we will clash again. When that time comes, show me what it is you fight for."

Without another word, the two turned and vanished over the wall. Aether tried to rush forward, but was stopped by more Hilichurls appearing. The two were quick to finish them off before chasing after their siblings. When they got to the other side, however, they only discovered that they were long gone.

Aether fell to his knees, tears building in his eyes. His sister was right in front of him.

She had been so close and he could reach her!

He bowed his head, hoping to hide his tears. He barely reacted when Diluc knelt next to him, pulling him into a hug. The other man had told him about what happened between him and his brother. How they had fought and he had left for three years, only to come back and find out that Kaeya had disappeared not long after while out on a scouting mission. He had been presumed dead for nearly four years.

He could only imagine how the older man felt.

The two sat there in silent solidarity as they grieved for their siblings.

What were they going to do now?

Once they were somewhere safe, Lumine immediately collapsed to the ground. Her wails were so loud that Kaeya was honestly surprised that none of the Mages had come running to see if she was okay.

Kneeling down next to her, he pulled her into a tight hug, letting her soak his shoulders in tears as he rubbed a soothing hand on her back.

"Shh," he whispered, "it's okay. Just let all those tears out. It's okay."

Her crying continued for what felt like forever and Kaeya felt his heartache for the girl who had taken him in. After Diluc had left, he had felt despair like he had never thought he could. He had been lost, left without a purpose and without a shadow to hide in.

Then he had met Lumine.

It hadn't been the smoothest of meetings – him having slaughtered several Abyss Mages before she had shown up – but it had ended in her holding out her hand to him, giving him a promise of family and hope. He found himself unable to turn her down and had sworn to himself that day that he would be her shadow. Swore that he would protect her like he couldn't protect Diluc and Crepus.

To see that same girl reduced to tears was heartbreaking.

He understood how she felt. They both knew the prophecy and how things would turn out, but they knew it needed to be done. Someone had to be the villain for there to be a hero. They would succeed in their mission against the Unknown Goddess and the heroes – Aether and Diluc – would prevail and become the heroes of Teyvat.

But until then, in this private moment, they could just be themselves.

A/N: I tried to start this yesterday, but I only got a couple words in before my brain decided that it didn't want to listen. So, technically, this story was written and finished today on the 24th of February. I just wanted to write something for my The Abyss Prince AU that I have on my Tumblr.

Basically, Kaeya gets adopted by Lumine not long after Diluc abandons Kaeya, he runs into Lumine, he sees a kindred soul in him and takes him in, making him the Prince of the Abyss Order. Kaeya and Lumine see each other as siblings, but don't use them to replace Aether or Diluc and still care about them.

If I ever write anything for it, the events in this story might not wind up being canon.