I'm basing this off of Kara 2x07, when Lilian Luther takes Kara and Mon-El. I do not own Supergirl or any of the show's characters. All rights go to their proper owners.
"Supergirl," the formal looking woman marched confidently into the room. "I don't believe we've been formally introduced."
The young alien pressed her face to the bars of the cage that held her prisoner. "I've seen you before," she said, narrowing her eyes at the older woman. "In Lena Luthor's office. What were you doing there?"
"Well," the woman exhaled, "I might ask you the same. I don't like the idea of you around my daughter." Her mouth pulled into a slight smirk as she stared down Kara.
Kara exhaled, the realization dawning on her. "You're Lilian Luthor, Lex and Lena's mother."
The woman stood straight, meeting Kara's eyes. "I'm a lot of things; a doctor, a patriot, a mother-"
"A kidnapper, a killer," Kara finished, her voice rising. "Does Lena know about Cadmus?" She searched Lilian's eyes, fierceness in her tone. "Does she know who her mother really is?"
Lilian narrowed her eyes. "And what are you to my daughter?"
"I'm a friend," Kara's tone didn't waver.
"Huh," Lilian scoffed. "I've heard that before." She tilted her head, walking around the cage that contained Kara. "Years ago, Superman came to my son Lex and promised him the world; a new dawn for mankind. Lex believed him, poor thing," Lilian sighed, her head down, but her body still straight, commanding attention from the alien.
"Then he watched," Lilian placed her hands on the metal bars, "as your kinsman infected this country with his propaganda as his power became unchecked."
She shook her head to herself. "And when Lex tried to take that power back, Superman convinced the world that Lex was evil," her tone hardened and she placed her hands on her hips. Shaking her head, she looked at the ground.
"My darling boy, my genius son, an actual superman behind bars for life." Lilian picked up her head, glaring at Kara. "Because you and your kind will stop at nothing from poisoning the earth," she spat.
"You're out of your mind," Kara scoffed.
"We'll agreed to disagree," Lilian said, completing her stalk around the cage.
"So what am I doing here?" Kara asked as Lilian walked passed her again.
"I need something from you," Lilian said. "I need you to be human for a little while."
Kara's eyes narrowed as Lilian continued. "I know from your fight with Red Tornado that when you expel enough of your heat vision, you deplete the solar radiation in your cells, leaving you as helpless as any mere mortal." She gestured to the door at the back of the cement room, and a man came in carrying a headpiece. It was a metal band with wires connected to a clear visor, something clearly of high tech.
"I've designed this helmet to absorb the radiation," Lilian motioned to the helmet as she spoke to Kara.
Kara scoffed, placing a hand on a metal bar. "You want me to solar flare?"
Lilian tilted her head, a small smirk pulling the corner of her lips up. "That's never going to happen," Kara said, yanking her hand off the bar and stepping back.
Lilian gave Kara a fake smile. "I can't hurt you," she said, turning away from the girl and walking around a guard. She inhaled, "But," she pulled a gun from the holster on the man's thigh, "I can hurt him," she said and shot Mon-El right above his knee.
Mon-El yelled, immediately grabbing his leg and falling against the cement.
"No, Mon-El!" Kara yelled, rushing to the bars that kept her from Mon-El. She watched him slam up against the back of the cage, grunts coming from his lips as he struggled with the pain.
"You know, we've been studying your friend," Lilian continued as if nothing had happened. "He's powerful, but he has a very bad lead allergy," she said, propping the gun in the air, an elbow on her hip.
Kara gasped, her breathing heavy, but she glared at Lilian nonetheless.
"Say you'll do it and I'll let him live," but Kara shook her head, refusing to give in. "Or," Lilian continued, "I can just put one in his heart right now," she snapped as she pointed the gun at Mon-El once again, who was still on the ground.
"No!" Kara gasped. "Okay, okay I'll do it! Stop!" She yelled, her voice frantic.
"Please," Mon-El gasped from the floor, "don't."
"I'm not gonna let you die," she said sternly, walking towards Lilian in the cage. She glared at Lilian through the bars. "Give me the helmet."
Kara stood in front of Lilian, who couldn't keep a smirk off her trained face. Lilian placed the helmet on Kara's head, and she knew what this would do.
If she's going to kill Mon-El, she thought, then there won't be anything I can do.
As the visor was placed over her face, Kara closed her eyes, taking a deep breath before she opened them, blasting the clear material with heat vision. The heat from her eyes hit the surface in front of her face, and immediately, the material seemed to grab onto her power, pulling it from her. It was intense, the pull awful. All Kara could do was keep her eyes open as the heat vision was sucked from her. The lights flickered and the room began to shake. Kara's body trembled as her strength was leaving her, her head snapping from side to side. She grunted, her hands clenched in iron fists against her sides, and the flickering lights began to pop, the bulbs exploding. The pull of the helmet as it absorbed everything Kara had left, was incredible and painful. Her whole body shook, rocking like she was being electrocuted.
"Aaahhh!" She screamed, expelling the last of her heat vision, her powers, and her strength. She wobbled on her feet, her eyes barely open. Kara felt the helmet being elevated from her head, and right as it was lifted, she fell. She crashed into the concrete floor, hitting her head off the ground, but she wasn't able to do anything to stop herself.
Lilian knelt down, a smile on her face. Gently she reached down to Kara and grabbed the back of her hair, yanking her face up to meet hers. Kara whimpered, the pain of Lilian's fingers grabbing her hair all too prominent now.
"Thank you," Lilian said. She released her grip on Kara's hair only to backhand her across the face.
Kara grunted, her head lashing to the side as she fell on her hands. Looking up at Lilian, Kara felt something warm running down her chin, and she could taste a deep metallic flavor on her tongue.
"Well," Lilian said, dabbing a finger at Kara's lip and looking at the red blood that dotted her index, "it worked." She looked up at the guards posted behind a fallen Kara. "Take her."
Kara felt the rough hands that pulled her up, but she didn't have the strength to stand, so they dragged her out of the room.
"Supergirl," she heard Mon-El's voice distantly as she was taken away. "Supergirl!"
