Meeting With "Iron Man"
Pietro and Wanda Maximoff were informed by Costel that Iron Man wanted to meet with them in the church. The twins went to the church and instead of finding Iron Man, they were meet by a stranger cladded in a red cloak who informed them of how much he enjoyed the church. The stranger then shed his cloak and revealed himself to be the Ultron, the artificial intelligence robot Tony Stark had designed and who had now gone rogue.
Forming An Alliance
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Ultron informed the twins that he planned to destroy the Avengers and that he needed their help to carry out his plans. Pietro and Wanda agreed to help aid Ultron because of their own hatred toward the Avengers and Tony Stark. The Twins then left the church with Ultron who proceeded to led the Maximoffs back to the HYDRA Research Base.
Ultron set up a machine capable of lifting an entire portion of Sokovia into the air, intending to plummeting it back onto Earth to cause a global extinction, with the machine's controller being located in the Novi Grad church. Iron Man arrived and confronted Ultron, who revealed his plan and activated the machine, prompting Iron Man to leave the church to assess the potential consequences. Later, Vision arrived into the church and briefly dueled against Ultron, erasing his presence on the internet so he couldn't escape through it.
During the ensuing battle between the Avengers and the Ultron Sentries, the fight between Ultron and Thor led them back into the church, where Thor dropped Mjølnir, which was picked up by Vision who used it to smash out Ultron. Iron Man then devised a plan to prevent a global cataclysm and asked the other Avengers to gather in the church, where they would protect the core against Ultron and its robots so that they could not cause it to fall back onto Earth.
Upon seeing this and being taunted by Thor, Ultron gathered all his sentries and had them charge the church. However, the Avengers were able to withstand the assault and destroyed countless androids without one of them being able to deactivate the repulsors keeping the part of Novi Grad in the air. Ultron himself joined the fight, but he was soon defeated by the combined forces of Vision, Thor and Iron Man before being punched away by Hulk, causing the Ultron Sentries to flee.
Scarlet Witch decided to remain in the church to guard the core while the other Avengers left to make sure that the city was evacuated. However, upon sensing the death of her brother Quicksilver, she left to confront Ultron, enabling one android to deactivate the repulsors, causing the city to fall back onto Earth. However, Iron Man and Thor overloaded the core in the church with enough power to shatter the whole meteorite, thus limiting the damage caused on Earth.