Standard disclaimer: I do not own the Avengers. Any characters and locations appearing in this fic are taken directly from the MCU or adapted from comics continuity unless otherwise noted.
A/N: This fic has been updated to mesh with canon up to the Vision's creation, at which point the timeline diverges (in this universe, the Vision's body exists, but the android as a sentient entity does not). There will be some spoilers. In order to comply with canon, some details in the first three chapters have been changed slightly ("Pym" has been changed to "Stark", the Maximoff twins exist, etc.) but most of this fic has been left intact. Thank you for reading.
Thor swung Mjolnir with a mighty cry, destroying one of Ultron's bodies, but another simply stepped up to take its place. Before the god of thunder could regain his stance, yet another snuck up behind him. Thor howled as the robot pulled its arm away, leaving a gory hole where the god's organs were meant to be. His hammer dropped from a suddenly slack grip. Thor fell to the ground and did not rise.
On the other side of the floating city, a hail of bullets tore through Pietro's back. The speedster dropped. Hawkeye pulled the child he had saved closer, but as he crept out a second attack cut him down as well. In the church, Wanda dropped to her knees, screaming in anguish.
Steve hurled his shield as, elsewhere, Ultron hit Hulk with something he'd developed based on one of Bruce's unused ideas that had been stored on Tony's servers. The projectile took out another robot, but Banner collapsed, human and unconscious. Steve reflexively caught the shield, and elsewhere Ultron stomped on Bruce's head, reducing it to a red smear. Shield in hand, Captain America charged, but Ultron was stronger and one of them finally got lucky. It crushed Steve's windpipe. Shield in hand, Captain America died.
Natasha dragged herself to the church on her probably-broken leg. Crushed robotic bodies showed evidence of Wanda's presence, but the girl was nowhere in sight. Against the far wall lay a single red and gold form. Tony had sacrificed himself to give the remnants of JARVIS enough time to block Ultron from the internet. He was trapped in a finite number of bodies now, but JARVIS and Tony were both gone. She activated her com.
"This is Black Widow. Does anyone read?" She was answered by silence. "Rogers? Barton? Anyone?" She tried again, her concussion blurring her sight so that she didn't see the single Ultron droid approaching the switch to drop the city. "Avengers, this is Black Widow. Come in, Avengers."
"They're dead." Wanda seemed intact physically, but her voice was empty. She dropped Ultron-Prime's heart drive with a clatter that echoed in the church. "All of them are dead, and so is my brother."
Natasha opened her mouth to say something, anything, but before she could think of the right words the Ultron unit activated the drop. The city sped downwards.
Wanda telekinetically tore the robot apart, but it was too late. She rushed to Natasha's side. "What do we do?" she asked, and Natasha remembered that she was just a child, really, who hadn't expected any of this. "How do we stop this?"
Natasha tried to focus, but everything hurt and she knew there was only one thing they could do now. She met Wanda's gaze. "I'm sorry, kid. You didn't deserve this."
"Tell me what to do."
"If this lands, the world ends. We can't keep it from falling, so we have to destroy it. I don't have the power for that, but you do. You can save the world, Wanda."
"But I won't survive." Natasha nodded slightly. She needed this to end. She needed to sleep. "I'll do it. Pietro was all I had. If I do this, I can join him, and my parents." As the room began to glow red, Natasha activated her com for the last time.
"Agent Hill, this is Black Widow. We failed to stop Ultron." Wanda fell to her knees. Her power spread over the city, reducing it to ash. "We're destroying the city, but there's no chance of evac. The Avengers are dead. Recommend you assemble a new team." The city crumbled, pieces breaking off and crashing to earth while those on the Helicarrier could only watch. "Good luck." With a final push, Wanda destroyed the city entirely. She and Natasha died with it.