I got great idea from a video I watched about Ateez as mythical creatures and Yeosang was a vampire. Also from the deja vu performance video(vampire ver.)
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Yeosang, a vampire lord of the vampire clan in Seoul, and who had been waiting for his mate for hundred years was making his way to the Blood Moon pack territory with a couple of his clan members. He was going to make an alliance with them to end the war between vampires and werewolves. He can see the alpha, Shin Sung Rok waiting for them at the entrance.

"Greetings, Lord Yeosang, it's a pleasure to have you here," Sung Rok greeted him.

"Greetings, Alpha Sung Rok. The pleasure is all mine," Yeosang greeted back.

"Right this way," Sung Rok said, leading him and his members inside the building. As soon as Yeosang got inside he smelt a lavender and bergamot scent. His eyes turned red. He knew it was his mate that he waited hundred years for. Yeosang followed the scent, ignoring everyone else. The scent led him to the basement.

In the basement

Kehlani was sitting on her bed, which is barely bed when she heard footsteps making their way down here. This has been her room ever since she was 13 years old, her second gender didn't present when she turned 12. She was called a runt, beaten and placed inside the basement. She didn't have any werewolf abilities, she was basically human. Her father and siblings didn't even care about her. Her mother died after giving birth to her. She lost too much blood to be able to heal. She does all the cleaning and cooking. She wasn't even fed. Her wounds wouldn't heal since she hasn't presented. She only got their leftovers, which wasn't much. She began to panic thinking she was going to get a beating when she heard the footsteps. No, no, no, I did everything, cleaned and cooked. What did I miss? Did they find out I took a loaf of bread? 

The door opened revealing Yeosang. Yeosang stepped into the room that had blood stains on the floor. He was disgusted. He looked around and saw his mate sitting on a bed with holes. She was rocking back and front in a panic, she was in a very bad condition. She was wearing ripped clothes, had wounds all over her body. She looked to be malnourished. He made his way to her slowly so as not to scare her even more. As soon as he started making his way towards her, she got up on her knees in a begging position in front of him.

"I'm sorry, Alpha. I didn't mean to steal a loaf of bread. I was just hungry. Please, forgive me. Please, don't hurt me," she begged, not knowing it was her mate, not Alpha Sung Rok. He kneeled down, he raised his hand to raise her chin, but she flinched.

"Don't worry, I won't hurt you," Yeosang said as he lifted her head by the chin. She had her eyes closed. She had a black eye and busted lips. Yeosang was angry at what he saw. "Please, look at me." She opened her eyes to see red eyes and a face she had never seen before. He smiled at her. She saw his fangs and panicked with tears in her eyes. I'm going to die. They sent a vampire to kill me! Yeosang heard her thoughts. It was one of his abilities as a pureblood vampire. He has six abilities, which you will find out a couple later.

"I'm not here to kill you," he assured her. His bite wouldn't kill her since she is his mate, which he was wondering how she doesn't know that he is her mate yet. Did he hear my thoughts or something? She thought. He decided not to tell her that just yet.

"You're not?" She asked, looking at him. He nodded, "Why would I kill my mate?" When he said mate, her eyes widened.

"You're my mate?" She asked. He nodded, caressing her cheek. She leaned into it. She was happy her mate was here. He will take her away from here. He smiled when she leaned into his touch.

"What's your name?"

"Kehlani," she replied. Yeosang was going to replied, but was interrupted by the so-called alpha.

"Lord Yeosang, what are you doing with the runt?" Sung Rok asked. As soon as Kehlani heard the alpha voice, she flinched away from her mate. My mate is friends with the alpha. He won't take me away from this place. Yeosang heard her say. He turned around and growled at Sung Rok, who flinched back.

"What did you call her?" Yeosang hissed. He knows what runt was, and he didn't like his mate being called that.

"Um, runt," Sung Rok answered in fear.

"Tell, Sung Rok," Yeosang said as he stood up and walked to Sung Rok, "Why is she here?" Sung Rok was scared, but held his ground.

"Um, well, she is a weakling, who hasn't presented yet. I don't allow weaklings in my park," Sung Rok answered nervously. Sung Rok snarled at him.

"So, you beat, starve, and lock her up here," Yeosang growled, not happy with his answer. Why does he cares about that stupid runt so much? He just met her. She will be the reason why we don't do the alliance. I should've killed her when I had the chance, Sung Rok thought, not realizing that Yeosang can read minds. Yeosang grabbed him by the neck and lifted him up. Sung Rok struggled to get free. Kehlani was shocked. She hated the alpha, but he didn't want him to be killed. She didn't know how to stop Yeosang. She started feeling this tingly feeling inside of her.

"Call his name," a voice inside of her said. She didn't know who it was, but decided to listen to it if it would stop a bloodshed from happening.

"Yeosang," She whispered, her voice traveling to Yeosang's ear. He stopped, then dropped Sung Rok. He turned around to his mate with red eyes. He was shocked to see her eyes were red. Why is he looking at me like that? Oh my god, I didn't put lord in front of his name. Yeosang smiled at her and walked back to her. He kneeled down in front. He held her cheek.

"Do you know your eyes are red?" Yeosang said to her. She looked at him in a really gesture, touching her eyes. Yeosang nodded. Sung Rok saw from where he was sitting and was shocked, she is an alpha! The reason they were shocked was that females don't ever present as alphas. There hasn't been a female alpha in a thousand years. Female alpha males are stronger than a regular alpha. Yeosang turned back to Sung Rok.

"Well, she's not a weakling after all, she's even stronger than you," Yeosang said to him, smirking. Kehlani started to smell a forest and ash scent. She knew it was coming for her mate. She saw that her wounds were slowly healing.

"She will be our seal of alliance," Yeosang said, "I will do the alliance for her sake." Yeosang picked up his mate. Kehlani wrapped her hands around his neck. He stopped beside Sung Rok.

"I should kill you and your pack for causing harm to my mate, but I won't since she doesn't want that. You should thank her for being alive right now," he said to Sung Rok, before walking out with his mate. All the pack members saw that the runt was with the vampire lord. They can smell the scent of a strong alpha. They just assumed it was their alpha, who must have gotten some powers from the lord from the alliance.

"Are you going to kill her?" A kid asked him, before his mother grabbed him.

"Of course, he will. Who would want a runt?" The alpha son said, smirking. Yeosang got irritated by him and froze him with his ice ability. Ice powers was another of his abilities. Everybody panicked seeing their future alpha frozen. Sung Rok came outside to see his son frozen.

"I'm sorry about him, please unfreeze him," he begged Yeosang. Kehlani looked up from her mate's neck to see the alpha's son frozen, making everyone see her red eyes.

"She's an alpha!"

"There hasn't been a female alpha in a thousand years." Whispers were heard from the pack members.

"He will unfreeze in an hour," Yeosang said, "Yes, she is an alpha. Also, my mate. Because of your foolish actions, you will lose your strongest force."

"My daughter, Kehlani," Kehlani's father said, stepping forward. Kehlani turned to look at her so-called father. Her red eyes shined brighter and she growled at him, her wolf took over.

"Daughter! Now that I presented as an alpha, you considered me as your daughter! Don't make me laugh!" She shouted angrily at her father, "You are not my father! You lost that the moment you turned your back on me!" She turned her back to him, letting Kehlani take over. Her father was shocked by her. Yeosang smirked.

"Hold on tight," Yeosang whispered into Kehlani's ear, who tightened her hold on his neck. Then, he used his teleportation ability. They disappeared.

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I just used a random Korean actor who played an evil character to played the evil alpha, haha.