Crossbones (Brock Rumlow) is a fictional character, a supervillain in the Marvel Comics universe. He was created by writer Mark Gruenwald and artist Kieron Dwyer in Captain America #359-360 (1989). Crossbones usually appears as a henchman of the The Red Skull. He carried out the assassination of Captain America, although Sharon Carter, who was under hypnosis at the time, is believed to have fired the fatal shots.
Crossbones assembles the Skeleton Crew from the Skull's henchmen and becomes their leader, searching for the missing Red Skull with them. They battle the Black Queen and her Hellfire Club mercenaries. Crossbones enlists the aid of psychic Tristam Micawber to locate the Skull. Upon finding him, Crossbones takes the Skull to Skullhouse for convalescence. Crossbones always remains loyal to the Red Skull, even tracking him down when he is imprisoned and is left to die by Magneto.
He next attends AIM's weapons exposition. He battles Daredevil during a failed assassination attempt against the Kingpin. He battles Bullseye during Bullseye's failed assassination attempt against the Red Skull. He battles Captain America again and is bested in combat. He later recounts how he met the Red Skull. The Red Skull assigns him to discover who killed the Skull's spare clone bodies. Alongside the Skeleton Crew, he battles the Schutzheilligruppe in an attempt to rescue the Red Skull and is captured. He is rescued from the Schutzheilligruppe's custody by Arnim Zola's ersatz Avengers. Crossbones is eventually fired for questioning the Red Skull's decision to ally himself with the Viper.
Desperate to regain his position as leader of the Skeleton Crew, Crossbones kidnaps Diamondback, imprisoning her in an abandoned subway station and forcing her into a brutal regime of combat training. Crossbones believes that he has brainwashed her into betraying Captain America, but Diamondback is actually laying a trap for Crossbones. She steals samples of Captain America's blood from the Avengers' mansion, then accompanies Crossbones to the Red Skull's mountain fortress. The two are captured and imprisoned, and Crossbones is rehired by the Red Skull on a temporary basis. He is later attacked by Cutthroat, new leader of the Skeleton Crew, who fears that Crossbones will try to take back his position as the Red Skull's right-hand man. Crossbones kills him, never realizing that Cutthroat is actually Diamondback's older brother, Danny. Crossbones later stabs Diamondback during her escape attempt, only to later save her with a blood transfusion in order to use her as bait. Ironically, he is critically wounded by flying shrapnel during an assault on the fortress by Captain America and Falcon, and imprisoned.
After he escapes from prison, Crossbones hires himself out as a mercenary and assassin to a number of villainous organizations, until he is rehired by the Red Skull, who is later apparently assassinated by the Winter Soldier. Since then, Crossbones and his new lover, Synthea Schmidt (the Red Skull's daughter), have been hunting the Soldier's commander, Aleksander Lukin. They plot to crash a stolen World War II era plane into the new Kronas Headquarters in London, only to have their plane destroyed by Red Skull's Sleeper robot. While they escape the destruction of the plane, they find Agent 13, and are about to kill her, only to be stopped by the appearance of the Red Skull. They are currently working with Red Skull/Lukin, whose minds both share Lukin's body.
During Marvel's Civil War event, Captain America and his Anti-Registration heroes surrender to Iron Man and his Pro-Registration heroes. While being led from Federal Courthouse, Captain America is shot in the shoulder by Crossbones, who is taking orders from Red Skull. Crossbones tries to escape in a helicopter but is tracked by Falcon and the Winter Soldier. Winter Soldier then beats Crossbones into unconsciousness, while Crossbones simply laughs in his face. Falcon then turns Crossbones over to SHIELD custody. Most recently, in Fallen Son: The Death of Captain America, Wolverine along with Daredevil and the help of Doctor Strange, broke into S.H.I.E.L.D to interrogate Crossbones and threatened to kill him. Crossbones revealed no knowledge of his hiring by the Red Skull. Wolverine leaves him a bloody mess on the floor after being convinced by Daredevil to spare his life.
S.H.I.E.L.D Director Tony Stark arranged for Professor X to scan Crossbones' mind for information, but Professor X found that someone (implied to be Doctor Faustus) had erased several parts of his memory in order to prevent such a scan. Sin and a new incarnation of the Serpent Squad have broken Crossbones free of S.H.I.E.L.D custody. They then capture Winter Soldier when he confronts Lukin to find out his relationship with the Red Skull.