The Rust Devils scavenge throughout the Commonwealth for functional robots and parts, which they reprogram and heavily customize; they are used for raids or base defence. Any robots they cannot capture or reprogram are stripped down for either parts to maintain their existing robot cohort, or to build armor for themselves. The Rust Devils are openly hostile to everyone, including other raider gangs; Rust Devils and raiders are seen fighting each other upon the Sole Survivor's arrival to the General Atomics factory.
During the quest Headhunting, the Sole Survivor must enter the Rust Devils' base at the Fort Hagen satellite array in order to get to one of the Mechanist's robobrains, Jezebel. Their leader Ivey and numerous Rust Devil raiders and robots are killed, as well as a heavily modified sentry bot named Ahab. Their presence is largely minimal from then on, limited to their robots ambushing caravans and trying to take down random robots across the Commonwealth.
Rust Devil members are simply displayed as Rust Devil, but their stats and equipment vary randomly based on the player's level. Legendary Rust Devils simply appear as a Rust Devil with a star beside their name.
Rust Devils can be encountered randomly across the Commonwealth fighting against various factions as well as occasionally launching attacks against player settlements. True to their specialization, they seem to have a higher chance of spawning at locations where robots are scripted to be present, like the outside of Cabot House, some of the pre-War military checkpoints scattered across the Commonwealth or player-owned settlements that host automatron companions.
The Rust Devils' robots are a surprisingly reliable source of top-tier ammunition types like fusion cores or mini nukes. Chances are high at least one robot in any encounter will drop one of those items upon death, regardless of robot type and even without any points invested in the Scrounger perk.
Rust Devils may be a problem for new players or low levels, due to them spawning in at Level 15 - they can become a major threat along with their robots for anyone unaware or ill prepared.
People settling in Starlight Drive In or Red Rocket may want to be cautious of the nearby random spawns as the Rust Devils commonly spawn with their robots and may venture into the settlements.