The Battle of Seoul was a major battle between the Avengers and Ultron with his sentries in Seoul, South Korea.
Having experienced nightmares from Scarlet Witch, the Avengers recuperated at Clint Barton's Homestead. Eventually, they realised 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. Realising 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, and Ultron killed the U-Gin employees and injured Cho.
Captain America arrived and Cho warned him about the Cradle and the Infinity Stone used to create the body inside.