Crossbones is a supervillain in the Marvel Comics universe. Crossbones usually appears as a henchman of the Red Skull. Crossbones has extensive training in street-fighting, military combatives, various forms of martial arts, and is a student of the Taskmaster.
Marvel Cinematic Universe
Brock Rumlow was a HYDRA agent within S.H.I.E.L.D., and a STRIKE team commander. He is a world-class hand-to-hand combatant, with extensive experience in various street fighting, martial arts, and military combat techniques. During the HYDRA Uprising, Rumlow was ordered to lead the HYDRA team and assassinate Captain America, until Rumlow was nearly killed during the Battle at the Triskelion, gaining horrific facial scars.
Once he recovered, and hearing that Alexander Pierce had died, Rumlow escaped from the hospital and, assuming the codename Crossbones, became an independent terrorist, setting out on a path to exact revenge and make anyone involved in his downfall suffer the same pain and loss he endured. He lured the Avengers into a public fight in which he personally fought Captain America. Although he was defeated, Crossbones activated a bomb hidden within his suit and, despite Scarlet Witch's best efforts to contain the blast, killed himself and dozens of innocent people in the resulting explosion.