The E522 assassin droid was a large, hulking construct laced with heavy weapons and equipped with thick military armor. It had a rounded, diamond-shaped torso with no noticeable head, and a very narrow waist that could rotate 360 degrees, two heavy tank treads that were covered in armored skirts, and two long and multi-jointed arms that were attached to armored shoulders. The arms were equipped with a heavy repeating blaster, heavy ion blaster, and a rapid-fire needler gun rather than hands.
Three smaller arms with highly nimble hands that were mounted low on the droid's torso and a was mounted on the back. The E522's body was covered with an assortment of sensors, antennae, and photoreceptors which gave the E522 a full 360 degree view, which worked in combination with the powerful internal combat computer and targeting system.
The E522 assassin droid was initially designed by Baktoid Combat Automata as part of their B-series battle droid line to serve as a light anti-armor and anti-aircraft unit for B1-series battle droids and B2-series super battle droids, but were too expensive and complex to serve as disposable units, and the project was scrapped in favor of B2 super battle droids and B3 battle droids. After the Clone Wars ended with the dissolution of the Confederacy of Independent Systems and the transition of the Galactic Republic into the Galactic Empire, the Techno Union was dissolved, and its subsidiaries either became independent or were nationalized by the Empire.
Baktoid Combat Automata was nationalized by the Empire, and immediately planned for breakup. Through the process of selling BCA's assets, all data from the E522 project was given to Sienar Intelligence Systems. At the time, SIS was developing its own line of combat and assassin droids, and the addition of the E522 and its data was a large boost. SIS immediately scrapped its own project and invested large amounts of time and capital into launching the E522 project. After a year of development, the first E522s launched from SIS's assembly line and brought into service to combat the growing unrest against the Empire.
By the time of the Galactic Empire, there were fewer than one hundred E522 droids in operation throughout the galaxy. Most were operated by Imperial Army Intelligence and the Imperial Security Bureau as assassins and support units for black ops. The droids were constantly memory wiped and received maintenance, giving them silent and terrifying personalities. A few E522s were in service to the Alliance to Restore the Republic, mostly working in the role of battlefield salvage and roles similar to the Empire's. Rebel E522s in Rebel Alliance Intelligence Service developed cynical and demanding personalities, so much so that agents described them as "everyone's least favorite Master Sergeant." However, Rebel E522s were well liked and respected for their bravery and competence during missions.
- Cyphers and Masks