Rust Devils are what happens when you combine immense technical aptitude with raider savagery. The Rust Devils are a recent arrival in the Commonwealth, who have settled at the Fort Hagen hangar. Using the immense supply of spare parts provided by pre-War military stocks, the Rust Devils have reinforced their numbers with a large number of restored robots of all grades. they continue to scour the Commonwealth for supplies, food, and most importantly, new robots. Their primary goal is to capture and reprogram robots, but if they cannot do so, they will happily junk the robot and strip it for spare parts to use in their ever-growing army of military-grade robotics.
The Rust Devils are obsessed with robotics. Apart from the aforementioned army of heavily customized robots, ranging from the humble Protectron to the might sentry bot (each heavily customized with cosmetic and functional modifications), the Rust Devils use salvaged armor plating as armor and junked robot weapons as handheld armaments (frequently without consideration for their own safety, as is the case with the salvaged assaultron head). Among their greater chievements is the Ahab, a custom sentry bot, and the captured Robobrain Jezebel - a prize taken from the Mechanist. The Rust Devils are more technologically advanced than the average Raider gang, and often use robots to fight their battles. They try to scavenge and reprogram robots before the Brotherhood of Steel can claim them. The Rust Devils also experiment with Human and robot augmentation. They use Human brains and tissue to control robotic bodies with mixed, and often terrifying, results.
Of course, the Devils are also commonly encountered across the world map in random encounters. They move in groups supported by robots and can be encountered fighting any other raider group, Gunners, rogue robots, and even super mutants. The proximity of spawning points to certain settlements, like Starlight Drive In or Red Rocket truck stop may also cause them to attack those, leading to a reliable source of robot parts, gear, and even fusion cores and mini nukes.
↑The Sole Survivor: "Who or what are the "Rust Devils?"" Ada: "Allow me to explain.My old caravan encountered them once while crossing the river into the city.The Rust Devils are a group of ruthless Raiders who utilize robots for their nefarious practices.They scour the Commonwealth looking for my kind. What they can't capture and reprogram, they strip for spare parts.I realize they pose a greater threat to me than you... I just want you to be well informed.That being said... Despite what lies ahead, I can easily continue to decrypt the signal as we proceed, so shall we be on our way?" (Ada's dialogue)
↑Automatron loading screen hints: "A new player in the Commonwealth, the Rust Devils are a Raider gang with a taste for technology and the smarts to use it. They frequently destroy robots just so they can utilize their scavenged parts."