Ultron moved his mind to HYDRA base in Sokovia after attacking the Avengers. Here, Ultron activated the dormant robots for the purpose of using them as means of transport if it was defeated by an Avenger. During the following days, Sentries and the Maximoff twins broke into different facilities stealing materials that Ultron needed to "save" the world.
A group of sentries traveled with Ultron and the Maximoff to Salvage Yard after find out the location of Ulysses Klaue, an arms dealer who possessed a big quantity of Vibranium. Eventually, the Avengers discovered the location of Ultron, which led to a battle between the heroes and the rogue robot. During the battle, many sentinels were destroyed, but one in which Ultron had transferred his memory in case of a possible defeat.
Having achieved the Vibranium, Ultron and his sentries traveled to Seoul, South Korea and confronted Dr. Helen Cho. Ultron ordered her to build a new body for him but as Cho refused, a Sentry used Loki's scepter to take control over her mind. When Helen Cho was released from the mind control with the help of Wanda Maximoff, the sentries killed the laboratory personnel. Knowing that the Avengers were heading to the lab, Ultron took his new body and fled in a truck belonged to the laboratory. To the misfortune of Ultron, the Avengers were able to snatch his new body, but he could capture Black Widow.
Days later, the Sentries attacked Sokovia and its population for Ultron's orders. However, the Avengers came to the city and easily defeated the robots, although they were surpassed in number. Many sentries went at the center of the city to stop the key and thus allow Sokovia fall against the Earth. After several attempts, a damaged sentry managed to stop the key after this was unprotected. Despite the key stop, Thor and Iron Man were able to destroy the city and prevent the extinction of the human race. Later, Vision confronts the last sentry and asked if he was afraid to die. They discussed the inevitable end of the human race and that Vision was unbearably naive. When the talk was over, Vision destroyed the sentry.
Marvel vs. Capcom series
The Ultron Sentries, called Ultron Drones in this game, appear as assistants for both Ultron in his standard form, as well as his merged form with Sigma, Ultron-Sigma, and are used extensively in the final battle against Ultron-Sigma's second and final form, Ultron Omega.