The Battle of Seoul was a major battle between the Avengers and Ultron with his sentries in Seoul, South Korea. Desiring a new body for his consciousness, Ultron breached U-GIN's Genetic Research Facility to form a new vibranium synthetic body in U-GIN's Regeneration Cradle.
When Scarlet Witch realized Ultron's actual plan was to cause a global catastrophe that would usher in a new age of Earth's evolution, the Avengers arrived, led by Captain America, and chased Ultron's U-GIN Truck across the streets of Seoul. Although the Avengers took custody of the Cradle, Ultron kidnapped Black Widow.
Escape from U-GIN
Having experienced nightmares from Scarlet Witch, the Avengers recuperated at Clint Barton's Homestead. Eventually, they realized that Ultron was attempting to create a new body in an attempt to "evolve." Ultron brainwashed U-Gin Geneticist Helen Cho, using the Scepter, and had her build a new body out of cells and the Vibranium Ultron stole from Ulysses Klaue. Realizing this, the Avengers split up in an attempt to stop him, with Tony Stark going to Oslo, Norway to keep Ultron from the internet and Captain America, Black Widow, and Hawkeye going to Seoul to stop Ultron as he attempted to upload his mind into the new body.
During the upload procedure, Scarlet Witch was able to read Ultron's mind, discovering his plan to destroy humanity. Before leaving Ultron, Scarlet Witch freed Cho from the Scepter's control, leading Ultron to kill the rest of U-Gin employees and left Cho severely injured. When Captain America entered the U-GIN lab, he arrived to see Cho on the ground and was warned about the Regeneration Cradle and the Infinity Stone that Ultron is planning to use to create the body inside.
Hawkeye managed to locate Ultron in a U-GIN Truck. Hawkeye suggested shooting it, but Captain America pointed out the possibility that the Infinity Stone inside Ultron's synthetic body could destroy the city. Rogers jumped on the truck, interrupting Ultron's download. Rogers decided to keep Ultron busy so that Hawkeye and Black Widow could retrieve the body. Rogers and Ultron fought, with Rogers doing his best to hold his own. At one point in the battle, Rogers lost his shield. Following the battle on a motorcycle, Black Widow was able to retrieve it for him. The fight eventually spilled into a moving train when Rogers tried to drawn Ultron away from the Cradle.
Meanwhile, Romanoff entered the truck; however, the Ultron Sentries lifted the truck into the air to evade a road blockade set up by local authorities. On the train, Ultron almost defeated Rogers when Scarlet Witch and Quicksilver came to Rogers' aid. Ultron begged the twins not to turn against him, but Scarlet Witch pointed out that they had no choice. Ultron killed the conductor of the train and fled. This caused the train to speed out of control. Rogers had Quicksilver move civilians out of the way while Scarlet Witch used her powers to successfully stop the train. Romanoff managed to roll the Regeneration Cradle into the Quinjet in midair, but she was kidnapped by Ultron in the process.
Repairs began in Seoul following the incident and continued to as late as May 3, 2016 when Seoul was among the cities noted by WHiH News anchor Christine Everhart to be still "on the mend" as a result of the incidents involving the Avengers, in part due to their tactical response, as the issue of the Avengers' management and responsibility was being discussed.