I wrote this on Tumblr and thought I'd share it here. I don't own Tangled or it's Characters and I learned of the Dark Siblings AU from glowamber. When I read about this I got interested and I actually wrote my own take on that AU. I'd like to write a full story with chapters, but for now, I shall write a one-shot. Constructive Criticism is welcomed since this is my first time writing something like this. I also apologize if it is sloppy.

Also, to learn more about this AU, check out my Tumblr, which has the same username as here, as well as check out glowamber's tumblr.

"Father is going to be mad when he finds out what you're doing." Princess Cassandra warned her older brother.

It was the middle of the night in the Dark Kingdom, with most of the castle staff in their rooms for the night. The 13-year-old Princess herself was just preparing for bed when she noticed her older brother, Prince Eugene, was out of his room, sneaking through the halls of the castle. Curious, Cass decided to follow the 16-year-old prince and see what he was doing. It wasn't too hard to follow him with being noticed.

Eventually, she followed him to a large set of doors. She realized that those were the doors to the Moonstone Chamber. That's when she finally decided to make her presence known, she couldn't let him go into that chamber, their father, King Edmund, had strictly forbade the children from going in there.

The prince rolled his eyes. "He won't find out. Besides, don't act you've never been curious to see the Moonstone yourself."

"That doesn't matter." Cassandra stated. "Our father told us that the moonstone is dangerous and we need to stay away from it."

"Calm down, Cass, it's not like I'm actually going in to touch it." Eugene said. "I'm just going to open the door a bit and get a quick peak at the stone."

The prince was about to open the doors, but Cass put her hands on the Moonstone Chamber door to keep him from opening it. The siblings glared at each other.

"I said no." Cassandra declared. "Dad said the moonstone is dangerous."

"Dad says a lot of things. But a lot of things he says doesn't make sense." Eugene countered. "He says that moonstone is dangerous, and that its our job to protect the Dark Kingdom and the whole world from it. Then why doesn't he do anything to destroy it? Why keep it here? And why tell us that we need to guard it, but won't let us just take a look at?"

"Its not that simple." The princess tried to justify. She hated to admit it, but a lot of what her brother says makes sense. She did know that getting rid of the Moonstone wasn't simple, but she had to admit that she wondered why her father doesn't do more to try to get rid of it instead of guarding it.

"Maybe not." Eugene admitted. "But I bet it isn't as complicated as Dad makes it seem." Then he moved Cassandra out of the way and opened one of the chamber's door, just enough to get a look at the moonstone.

Cassandra groaned and looked away. Eugene made it clear that he wasn't going to listen to hear. She decided to just go back to her room and go to bed.

"Whoa…" Eugene sighed as he looked through the door. "Cass, you have to see this."

"No." Cass shook her head. "You're going to get in trouble for this and I won't get in trouble too."

"No one will know as long as neither of us say anything. I won't say anything if you agree not to say anything." Eugene argued. "C'mon, this could be our only chance to see it in a long time."

Cassandra continued to look away, not wanting to disobey their father. But she eventually gave into temptation and joined Eugene. She moved him out of the way to peak into the Moonstone chamber the crack of the doors.

What she saw took her breath away. The room itself was huge, it also very empty and dark, which made sense since no one really goes into it. But in the center of the room, far from the door, stood the moonstone opal, encased in a cage of black stone. Cassandra had always imagine the moonstone as an ugly and dark chunk of rock. But as she gazed at the mysterious stone, she had to admit that it was not what she expected.

The moonstone was glowing brightly, shining a blue light in the dark chamber. It also was a perfect oval shape, and although she couldn't tell, she imagined its surface was smooth without a single scratch on it. To think, this was the stone that their father had warned them against. How could something like this cause so much death and destruction?

Before Cassandra could ponder more, she felt a hand on her shoulder and pull her away from the door. She felt herself freeze as she realized who it was.

"Mother!" Cassandra gasped. "I can explain! I-"

"I don't want to hear your excuses, Cassandra!" Royal Consort Gothel declared. "You know you shouldn't be here! Your father has told you countless-"

"It wasn't her fault!" Eugene defended, stepping in between Cass and Gothel. "I was the one who came to peak at the moonstone, Cass was just trying to stop me and I tricked her into looking at the moonstone with." He added as he shielded his sister from Gothel.

Gothel looked at the two teens before taking a deep breath and giving Eugene a sweet, affectionate smile. "Eugene, dear, you know better than that. You really need to a set a better example for Cassandra. Not just as her older brother, but as the future king of the Dark Kingdom."

"I…I know, Gothel." Eugene nodded. Eugene had to keep himself from vomiting as Gothel kissed his forehead.

Gothel wasn't Eugene's mother. When Eugene was a very young, his mother, King Edmund's first wife and the Queen of the Dark Kingdom, died. His father never told him how she died, for the topic of Edmund's first wife is always difficult for him. When Eugene was a toddler, his father had met and married Gothel, and soon, she gave birth to Cassandra, Eugene's half-sister.

Eugene tried to get along with Gothel, for his father's sake, but Eugene hated the woman. His father may have been blind, but the prince could see Gothel for what she really was: Manipulative, selfish, deceitful, and ridiculously vain. She would present herself as loving and doting to him and Cassandra, but Eugene knew better, he knew she was acting like that for her own selfish gain.

"Alright. I won't tell your father, I think you both learned your lesson." Gothel gently said. "Now, off to bed you two. Its getting late."

"Yes, Gothel." Eugene said.

"Yes, Mother." Cass quietly said.

Eugene was the first to leave the chamber, with Cassandra quietly following. But once Eugene was gone, Gothel put a hand on Cass' shoulder, and whispered into the child's ear.

"I hope you're proud of yourself, Eugene had to lie to defend YOUR actions." Gothel harshly whispered. "I better not hear him do that again, I won't have his future jeopardized because of you."

"Yes, Mother." Cass quickly whispered. Then Gothel let go of Cassandra and the princess immediately ran out of the Moonstone Chamber.

Cass runs back into her room and immediately jumps into her bed and cries into her pillow. It wasn't fair! Going to see the Moonstone was Eugene's idea, yet her mother blames HER! And doesn't even believe him when he tells her it was his idea! Why did her mother always blame her for when Eugene does something wrong?

"Cass?" A familiar voice whispers. Look up from her bed, she sees Eugene standing outside her bedroom door.

Cass sits up and quickly wipes her eyes so her brother wouldn't see she was crying. "What do you want?" She snapped.

"I wanted to make sure you were okay." Eugene says. "Gothel didn't hurt you, did she?"

"Why do you care?" Cass argued. "Once again, she blamed me for what YOU did!"

"I'm sorry." Eugene said. "I didn't think she of all people would catch us in the Moonstone Chamber. And I told her it was all my idea and that you were trying to stop me."

Cassandra scoffs. "She didn't believe you. She thinks its all my fault. She never thinks you, her favorite child, could do anything wrong."

"Don't call me that." Eugene states, sounding disgusted.

"Why not? It's true." Cass says, feeling more tears form in her eyes. "No matter what I do or how much trouble you get into, Mother prefers you. Its not fair."

Eugene sighed and sat beside Cass on her bed. "You know I don't like this anymore than you do, Cass. I don't try to get you into trouble, and I certainly don't try to be Gothel's favorite. I don't care about that. What I care about is making sure that witch doesn't hurt you. You don't deserve to be treated the way Gothel treats you."

Cass starts to cry again. She tries to wipe her eyes again, not wanting to cry in front of Eugene. Then she felt Eugene's arms wrap her in a hug. She tried to push him away, but eventually, she allowed herself to cry into his chest as he continued to hug her.

"It'll be okay, Cass." Eugene promises. "I won't let Gothel treat you like that."

Then he feels Cassandra push him away and off her bed. She glares at him. "I don't need you to protect me!" She snaps. "Its your fault Mother treats me like that at all. If you just stay away from me and not get me into trouble with you, everything would be fine!"

Eugene wanted to speak, but Cassandra continues to push him out of her room and slams the door when he's finally out.

Eugene tries to open the door, but its locked. "Cass, open up!" He says. But Cassandra doesn't respond or unlock her bedroom doors.

Realizing that she is mad at him and doesn't want to speak to him anymore, Eugene decides to leave her alone for the rest of the night. With a sad sigh, Eugene leaves and goes to his room for the night.

"I'm sorry, Cass…" Eugene whispers.