Ultron Sentinels

Ultron Sentinels are the secondary main antagonists in the movie Avengers: Age Of Ultron. They were created by Ultron who needed a number of minions of his disposal, something which foreshadowed by moments when he hijacked a number of Tony Stark's Iron Legion.

Mass-Construction
After barely escaped from his prototype body that destroyed by Thor, Ultron goes for HYDRA base at Sokovia through Internet and taking control over machines in there. With his stolen Iron Legion schematics and having studied the unfinished drones that still developed by HYDRA where their developements previously stopped by Avengers, he mass-produced the Sentinels whilst creating more durable body for himself. The first new robots then worked to resume the production whilst Ultron himself rendezvous with Maximoff twins.

When Ultron introduced them to Maximoff twins, there are already less than 20 Sentinels that busily mass produced more machines and more robots to amass an army.

Buying Vibranium and Creating More Organic Body
At least 4 to 5 Sentinels deployed at to accompany Ultron and Maximoff twins. Due to ensuin battle, two of them destroyed, but the rest managed to acquiring vibranium. 3 of them deployed again both to secure the cradle that contained the newly-built more organic body for Ultron. One of them was destroyed when two of them took the truck’s container away whilst left the truck itself crashes and destroyed the Sentinel whom drove it.

Final Battle and Destruction
Once the project to replicating the meteor commenced, a wave of them came to attack Novi Grad just as the Avengers arrived. Ultron would possessed some of them to directly communicate with the Avengers on several occasions during the battle. Finally,  the final wave of them, which are the overall remained Sentinels which estimated around thousands of them came to reach the device's trigger. Many of them were killed, but when Scarlet Witch suffers the breakdown over Quicksilver's demise, her powers caused the last surviving Sentinels destroyed simultaneously, though Ultron managed to possessed one to activate the device's trigger seconds before that robotic body became useless. Not long after his plans failed, Ultron used the last and remaining least fully-function body of this Sentinel to confront Vision before being destroyed.

Though many of them destroyed and most were reduced into scraps, some ended up salvaged and sold in black market.

Appearance and Capabilities
The Ultron Sentries bear a close resemblance to their Master, the primary differences being that they are less bulky, have less armor and are slightly shorter. They have a blue glow that emanates from within their bodies, that turns red when carrying Ultron's consciousness. Unlike Ultron, they are unable to shoot concussive beams from their fingers. Instead, they are able to transform their arms into cannons.

Trivia

 * They look similar to Ultron's appearance from the original Marvel Comic Books.
 * They are based off of the Ultron army from the original comics and Marvel: Ultimate Alliance.