Not much is known about the early life of the man that would become Crossbones. As a young man, Brock Rumlow was the leader of a small New York gang. There he encountered a young Diamondback, she wanted to join his gang and he used his size to overpower her and rape her. When she came home her two brothers saw what happened to her and went after Brock for revenge. One of the brothers was killed by Crossbones with a throwing knife and the other left the family in shame to eventually become the mercenary Cutthroat.
As he grew and became more a part of the criminal underworld he began learning about the Red Skull and looked up to him as a god-like figure. After joining Taskmaster's school for criminals, Crossbones worked his way up the ladder and quickly became a teacher. While observing some of his fights, the Red Skull instructed Taskmaster to bring Brock to him. Sometime later he began working for Albert Malik, the man who was claiming to be the Red Skull. He was hired to take a team of mercenaries and break into a castle belonging to the actual Skull. After avoiding defenses and showing some skill with a bow and arrow, a blob-like monster proved to be the ultimate threat. All but one other mercenary was killed and they were taken prisoner by the Skull and Arnim Zola. Brock was interrogated by the Skull and he revealed who his employer was and his dedication to the Red Skull. Rather than kill him, the Skull challenges Brock to a fight in order to test his new body that had been cloned from Steve Rogers. Exhausted from scaling the mountain to get to the castle, Brock is beaten but the Skull sees the evil in him and offers him training and a spot as his new bodyguard. He also gives him his new name, Crossbones.
The Skull's Assassin
Years later, Brock has become one of the world's greatest killers and is working for the Red Skull as the leader of the Skeleton Crew and his most loyal follower. After the Skull is kidnapped and tortured by Magneto during Acts of Vengeance, Crossbones leads the assault on the Hellfire Club's New York building to break him out. Once inside, Machinesmith hacks into their cameras and tells them that their leader isn't there. Crossbones then forces a weak psychic to track down the Skull. They find him near death in an abandoned bomb shelter hidden in a cemetery. He asks to be taken to Skull House and - despite protests from the rest of the Skeleton Crew - Crossbones takes him there. Crossbones sees his boss' condition as the result of Magneto destroying his killer instinct and he tries to help his boss by reminding him of all the people he has killed but it doesn't work. The Skull then asks him to summon his greatest enemy, Captain America, to Skull House. Upon seeing the face of everything he hates, the Skull regains his will to live so that he can kill Cap.
While recuperating, the Red Skull waged a war against the Kingpin. Crossbones was sent to kill the Daredevil villain. Crossbones attacks him in a restaurant but Daredevil interferes. Crossbones defeats him (and the hero is left pinned under a table), but during this, the Kingpin escapes. Bullseye then attempts to kill the Red Skull and battles Crossbones. Lester kills a robot of the Red Skull and the two fight in the room. Bullseye says he is impressed by Crossbones' speed, and Crossbones quickly closes the gap between the two and punches him once, knocking the assassin over. Crossbones then begins to choke him, saying he could kill him in a second, but he deserves to suffer. Bullseye then spits a fake tooth in his eye and stabs his bicep, then flees the building.
The Skull is kidnapped by Captain Germany and put on trial for his crimes as a leader of the Nazi party. Crossbones then leads the Skeleton Crew in an attempt to take him back but they are defeated by Germany's team of super soldiers and are put on trial with the Skull. Arnim Zola breaks them out using his genetically-altered soldiers to pose as the Avengers. After they were returned to the Smith Building, the Skull had all of the Skeleton Crew fake their deaths so that they could go into hiding and they could continue their business without government surveillance. The Red Skull decides to add fresh blood to his criminal organization and orders Crossbones to break Viper out of prison. Crossbones then voices his disapproval and is immediately fired for insubordination. In an attempt to get back in the Skull's good favor, he kidnaps Cap's girlfriend, Diamondback, and attempts to brainwash her into betraying him. He appears to succeed and she even steals a sample of Cap's blood from Avengers Mansion for him, but after he regains entry into the Skull's lair, he returns to the Skeleton Crew and Cutthroat begins to fear that he will be replaced by him and tries to kill him in his sleep. Unfortunately for Cutthroat, Crossbones was told of the plan by Mother Night, and he kills Cuttroat first (by slitting his throat), causing Diamondback to reveal that her apparent brainwashing is just an act and she tries to kill him in retaliation as Cap storms the Skull's fortress. They narrowly escape the destruction of the base and Crossbones is injured when an explosion sends shrapnel into his back.
The Death of Captain America
Crossbones was the sniper who shot Steve Rogers after the Civil War. He was taken into S.H.I.E.L.D. custody and interrogated but his mind had been erased of all information about the Red Skull by Doctor Faustus and he was useless even to the likes of Charles Xavier. The most recent incarnation of the Serpent Squad led by his girlfriend Sin was sent to rescue him. They succeeded but he and most of the serpent squad were captured and sent to prison shortly after by the new Captain America. Crossbones battled the new Captain America, and threw him out a window. Captain America was saved by Black Widow and shot Crossbones several times in the chest. In prison, he sat patiently and waited for some sign from the Red Skull until he heard a guard mention that they also had Sin in custody. Crossbones then killed a guard and went on a rampage searching for her. After knocking out multiple guards, he found her in the hospital and manages to kiss her before being gassed and returned to his cell. Norman Osborn has recently released Crossbones, saying in exchange he needs to work for him.
As a Thunderbolt
When the Heroic Age came, Crossbones found himself on the Raft. He was placed into the current Thunderbolts team, despite the protests of Luke Cage. The reason he is on the team is because Crossbones is generally an unlikable person, so if any of the inmate Thunderbolt members disagree with Cage, they might think twice because they'll be siding with Crossbones. Crossbones is also not permitted to have lethal rounds of ammunition on him, so he's carrying around tranquilizers and hard rounds only. He has currently completed the Troll mission, where he shot a human girl dressed up as a troll, and is the cave mission, where he equipped a mini-gun and flamethrower to defeat these mutant monsters.
When Crossbones and the other members were sent to New Guinea to investigate a deposit of Terringen Crystals, Crossbones was exposed to its radioactive mist. During the unsanctioned mission to rescue Darris Kimbro from the Hands underground fortress under Hell's Kitchen, it seems that Crossbones has developed powers as a consequence to his exposure of the radioactive mist from the Terringen Crystals. With his new-found powers of generating fire from his mask, he wiped out a whole squad of Hand ninjas, but also killed Darris Kimbro in the process. He was recently dropped from the team after an escape attempt was foiled by Steve Rogers, Iron Man, Thor, and Luke Cage. During the encounter, Crossbones had a brutal fight with Rogers, taking the edge and besting him with his new-found powers and hand-to-hand abilities. When Captain America was down, they were standing near water which revealed a reflection of who they want to be. Crossbones was shocked by seeing a reflection of the Red Skull, and the hero took advantage of the moment, defeating his foe. Later on, during the events of Fear Itself, Crossbones was able to escape The Raft and killing the man who helped him out.
Working for the Man
During his time in custody, Crossbones was sent to an isolated island that was hit by a terrible virus outbreak. Brock was sent in with one objective: rescue the one person who may possess the resistance to the virus. Heavily armed, Brock eventually meets a camp of survivors on the island. It turns out the individual is just a boy, and Brock betrays the survivors by letting the monsters (who are essentially werewolves) into their base. Brock battles a few, snapping their necks and withstanding a handful of claw attacks before reaching his evacuation helicopter with the child. But, Brock betrays his employers by pushing the boy out of the helicopter, sending him falling to his death.
After the events of Fear Itself, Sin is now in search of the missing hammers and other mystical weapons. As her lover and right-hand man, Crossbones is now globe-trotting to find these items. This journey has taken him to France, underwater, face-to-face with Namor, and, most recently, he took down Luke Cage while wearing one of the gauntlets he retrieved. Previews reveal this former Captain America foe will soon face-off with famous X-Man, Wolverine.
House of M
Crossbones was one of the many human villains that the Hood assembled which became that universe's incarnation of the Masters of Evil. He went along with all of the Hood's heists and was present in the rescue of their teammates, who were arrested by the Red Guard. After the rescue, he took part in the liberation of Santo Rico from the House of M. After Magneto sent his forces to retaliate, Crossbones left Santo Rico along with many others who did not want to risk their lives against the overwhelming odds of Magneto's forces.
In the battle for the liberation of Santo Rico, a conversation between the Hood and Crossbones in the heat of battle reveals that Crossbones might have been working for different black-ops teams. The Hood noticed that he might have done missions like this before, to which Crossbones replies that he hasn't done it in a while but it is hard to forget how to do it. He may have as well been a mercenary in this reality.
Powers & Abilities
Originally, Crossbones had no powers. However, Crossbones once developed powers as a consequence of Terrigenesis (exposure from the radioactive mist of the Terringen Crystals). These powers allow him to generate intense heat and fire them as beams at his enemies. However, the power vanished when his Thunderbolts teammates showed up. It may have been a subconscious reaction to keep his new powers secret or he may have just exhausted the power.
He possesses peak human strength, speed, and stamina. Daredevil once said Crossbones was difficult to predict in combat due to his high metabolism. He is a highly skilled hand-to-hand combatant, being able to go toe-to-toe with Captain America on multiple occasions. Crossbones was trained in combat by the Taskmaster; quickly advancing and becoming a teacher of combat at Taskmaster's school for criminals when he was just a teenager. He is also rather quick for a man of his stature. We've seen him dodge gunfire on various occasions, and Bullseye even remarked on his speed once. Crossbones is an excellent marksman with guns, crossbows, and throwing knives. He was able to tag Troll in the ear while she was lunging. He is experienced with torture and brainwashing methods. For instance, he once effectively "reprogrammed" Sin and almost did the same with Diamondback. Brock is an expert pilot, and has a very high degree of pain tolerance. We've seen him remain silent when an arrow stabbed him in the gut and he recently told Sin it'll "take more than a few bullets and missiles to stop me." He is tough enough to have laughed off Captain America's attacks.
Various firearms (preferred Weapons: Colt M1911's or Berrettas) and a bowie knife. He also uses a collapsible wrist crossbow, a wrist spike, spring-loaded stiletto blades in his gauntlets, explosives and poisoned knives.