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The mission was supposed to be simple. In and out. Retrieve the stolen files. No casualties. That's what Nick Fury had told the team before sending the Avengers to an abandoned HYDRA research facility hidden beneath the snowy forests of Sokovia. Explosions echoed through the underground halls as alarms screamed overhead. Steve's voice crackled through the comms. "Everyone fall back! We've got the intel!" You—18 years old and the youngest field agent S.H.I.E.L.D. had ever recruited—covered the team's escape, weaving through collapsing corridors. Then you heard it. A quiet hiss. Before you could turn around, a HYDRA scientist slammed a strange injector into your neck. A burning liquid flooded your veins. You gasped, stumbling backward as the scientist smiled. "It's already inside you." Natasha shot him before he could say another word. You ripped the injector out, your vision blurring. "I'm... I'm okay," you lied. The burning faded within seconds, leaving only a dull ache. The Avengers searched the lab for answers, but every computer had been wiped. Every sample destroyed. Tony scanned your blood. "Nothing unusual." Bruce frowned. "I can't find anything." So everyone assumed you'd been lucky. You wanted to believe them. --- Three Weeks Later It started with your hands. A coffee mug slipped from your fingers and shattered across the kitchen floor. You stared at your trembling hand. "...Weird." Then your legs began giving out during training. Bruises appeared from the smallest impacts. Your strength faded day by day. No matter how much you slept, you woke up exhausted. The scans still showed nothing. But deep inside your body... Something was spreading. Slowly. Patiently. Destroying you from the inside out. And somewhere in the shadows, a surviving HYDRA scientist watched a monitor displaying your medical data. He smiled. "Phase One... successful."