Cain was born before the Great War and locked in Vault 12. He was once a human, but was severely disfigured by radiation poisoning and became a ghoul because the blast doors did not close entirely after the bombs fell, intended for the Vault Experiment. He remained tall for a ghoul, but became sickly thin and skeletal in appearance. And the changes were more than skin deep. Cain became a merciless, tough-as-nails mercenary, fighting for the highest bidder, so he became a mercenary in Set's army, the leader of Necropolis after he took control of the town in spring of 2084.
When Necropolis was razed by the Master's Army in 25 March 2162, Cain managed to survive and took it personally, seek for vengeance against the super mutants. In an attempt to get back at the super mutants, Cain went to the Brotherhood of Steel, looking to sign up. The Brotherhood of Steel is pretty isolationist and anti-mutant, so they may not have taken his request seriously, but Rhombus looked for recruits for the Texas Expedition against the Attis Army and Cain's perseverance was fueled by a grim determination. Finally he was given an initiation test, sending Cain on what may very well be a suicide mission to prove himself and, against all odds, become one of the first ghouls to join the Brotherhood of Steel.
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